X.Y.Z.Sources nuget package (with pre-processor transformations support)

PackagePrep.csproj

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>netcoreapp2.1</TargetFramework>    
  </PropertyGroup>
</Project>

Program.cs

using System;
using System.IO;
using System.IO.Compression;
using System.Text;

namespace PackagePrep
{
    class Program
    {
        static void Main(string[] args)
        {
            if (args.Length == 0 || args.Length > 3 ||
                args[0].Equals("tokenize", StringComparison.OrdinalIgnoreCase) && args.Length != 3 ||
                args[0].Equals("nuspec", StringComparison.OrdinalIgnoreCase) && args.Length != 2)
            {
                Console.WriteLine("PackagePrep [tokenize|nuspec] [project-directory] (rootNamespace)");
                return;
            }

            Console.WriteLine($"PackagePrep {string.Join(' ', args)}");
            Console.WriteLine();

            void ReplaceFileInArchive(ZipArchiveEntry entry, string content)
            {
                using (var stream = entry.Open())
                {
                    stream.SetLength(content.Length);
                    using (var sw = new StreamWriter(stream))
                        sw.Write(content);
                }
            }

            string ReadFileInArchive(ZipArchiveEntry entry)
            {
                using (var sr = new StreamReader(entry.Open()))
                {
                    return sr.ReadToEnd();
                }
            }

            var command = args[0];

            if (command.Equals("tokenize", StringComparison.OrdinalIgnoreCase))
            {
                var path = args[1];
                var rootNamespace = args[2];

                Console.WriteLine("Tokenizing sources...");
                Console.WriteLine("Path: " + path);

                foreach (var file in Directory.GetFiles(path, "*.pp", SearchOption.AllDirectories))
                    File.Delete(file);

                foreach (var file in Directory.GetFiles(path, "*.cs", SearchOption.AllDirectories))
                {
                    var text = File.ReadAllText(file, Encoding.UTF8);

                    text = text.Replace($"{rootNamespace}.", "$RootNamespace$.");

                    var sb = new StringBuilder();
                    sb.AppendLine(text);

                    var code = sb.ToString();
                    File.WriteAllText(file + ".pp", code, Encoding.UTF8);
                }
            }

            if (command.Equals("nuspec"))
            {
                var path = args[1];

                Console.WriteLine("Correcting nuspecs...");
                Console.WriteLine("Path: " + path);

                foreach (var package in Directory.GetFiles(path, "*.nupkg", SearchOption.AllDirectories))
                {
                    Console.WriteLine("Found package: " + package);

                    using (var archive = ZipFile.Open(package, ZipArchiveMode.Update))
                    {
                        for (int i = archive.Entries.Count - 1; i >= 0; i--)
                        {
                            var entry = archive.Entries[i];
                            var file = new FileInfo(entry.FullName);
                            if (file.Extension == ".nuspec")
                            {
                                var before = ReadFileInArchive(entry);

                                Console.Write("Found .nuspec: " + file.Name + "...");

                                var updated = before
                                    .Replace("exclude=\"Build,Analyzers\"", string.Empty)   // fix PackageReferences
                                    .Replace("buildAction=\"Content\"", string.Empty);      // fix file tags


                                if (before == updated)
                                {
                                    Console.WriteLine(" not modified.");
                                }
                                else
                                {
                                    Console.WriteLine();
                                    Console.WriteLine(updated);
                                }

                                ReplaceFileInArchive(entry, updated);

                                var after = ReadFileInArchive(entry);

                                if (after != updated)
                                {
                                    throw new IOException("The update process did not change the .nuspec.");
                                }
                            }
                        }
                    }
                }
            }
        }
    }
}

X.Y.Z.Sources.csproj

<Project>

  <Import Project="Sdk.props" Sdk="Microsoft.NET.Sdk" />

  <PropertyGroup>
    <TargetFramework>netstandard2.0</TargetFramework>    
  </PropertyGroup>
  
  <PropertyGroup Condition="'$(Configuration)'=='PackageSources'">
    <IsPackable>true</IsPackable>
    <IncludeBuildOutput>false</IncludeBuildOutput>
    <ContentTargetFolders>contentFiles</ContentTargetFolders>
    <DisableImplicitFrameworkReferences>true</DisableImplicitFrameworkReferences>
    <GenerateAssemblyInfo>false</GenerateAssemblyInfo> 
    <GenerateTargetFrameworkAttribute>false</GenerateTargetFrameworkAttribute>
    <NoWarn>CS8021</NoWarn>
    <NoBuild>true</NoBuild>
    <GeneratePackageOnBuild>true</GeneratePackageOnBuild>
  </PropertyGroup>
  
  <ItemGroup Condition="'$(Configuration)'!='PackageSources'">
    <None Remove="**\*.pp" /><!-- Don't show generated files during regular development -->
  </ItemGroup>
  <ItemGroup Condition="'$(Configuration)'=='PackageSources'">
    <None Include="**\*.pp" Exclude="obj\**" />
    <Compile Remove="**\*.cs" /><!-- Don't show source files during package deployment -->
  </ItemGroup>
 
  <ItemGroup Condition="'$(Configuration)'=='PackageSources'">
    <Content Include="**\*.pp" Exclude="obj\**">
      <Pack>true</Pack>
      <PackagePath>$(ContentTargetFolders)\cs\netstandard2.0\$(ProjectName)\%(RecursiveDir)%(Filename)%(Extension)</PackagePath>
    </Content>
    <EmbeddedResource Update="@(EmbeddedResource)">
      <Pack>true</Pack>
      <PackagePath>$(ContentTargetFolders)\any\any\$(ProjectName)\%(RecursiveDir)%(Filename)%(Extension)</PackagePath>
    </EmbeddedResource>
    <PackageReference Remove="@(PackageReference)" />
  </ItemGroup>

  <Import Project="Sdk.targets" Sdk="Microsoft.NET.Sdk" />

  <Target Name="Compile" Condition="'$(Configuration)'=='PackageSources'" />
  <Target Name="CopyFilesToOutputDirectory" Condition="'$(Configuration)'=='PackageSources'" />
  
  <!-- Unfortunately the .nuspec file is not valid for .pp files due to the attributes added by None/Content/Compile -->
  <Target Name="PostPack" AfterTargets="Pack" Condition="'$(Configuration)'=='PackageSources'">
    <Exec Command="dotnet run --project &quot;.\PackagePrep\PackagePrep.csproj&quot; -- nuspec $(ProjectDir)bin\$(Configuration)" />
  </Target>
  
  <!-- Transform all .cs files into .cs.pp files with $RootNamespace$ -->
  <PropertyGroup>
    <PreBuildEvent>
      dotnet run --project &quot.\PackagePrep\PackagePrep.csproj&quot; -- tokenize $(RootNamespace) $(MSBuildProjectDirectory)
    </PreBuildEvent>
  </PropertyGroup>

</Project>

Thanks for sharing, just what I was looking for!