Automatic DependentUpon

DynamicDependency.csproj

<Project Sdk="Microsoft.NET.Sdk">
  
  <PropertyGroup>
    <TargetFramework>netstandard1.4</TargetFramework>
  </PropertyGroup>
  
  <ItemGroup>
    <PropertyPageSchema Include="$(MSBuildThisFileDirectory)ProjectItemsSchema.xaml" />
    <ProjectCapability Include="DynamicDependentFile" />
  </ItemGroup>
  
</Project>

ProjectItemsSchema.xaml

<ProjectSchemaDefinitions xmlns="http://schemas.microsoft.com/build/2009/properties">
  
  <ContentType Name="PageXaml" DisplayName="XAML Page" ItemType="Page">
    <NameValuePair Name="DependentExtensions" Value=".xaml.cs" />
  </ContentType>
  
  <FileExtension Name=".xaml" ContentType="PageXaml" />
  
</ProjectSchemaDefinitions>

StaticDependency.csproj

<Project Sdk="Microsoft.NET.Sdk">
  
  <PropertyGroup>
    <TargetFramework>netstandard1.4</TargetFramework>
  </PropertyGroup>
  
  <ItemGroup>
    <Compile Update="Hoge.*.cs">
      <DependentUpon>Hoge.cs</DependentUpon>
    </Compile>
    <Compile Update="Fuga\Fuga.*.cs">
      <DependentUpon>Fuga\Fuga.cs</DependentUpon>
    </Compile>
  </ItemGroup>
  
</Project>