Sets $env:PSModulePath Regardless of PS Version or Operating System

Set-PSModulePath.ps1

function Set-PSModulePath {
    [CmdletBinding()]
    Param ()

    [System.Collections.ArrayList]$ExpectedPaths = @()

    # Handles Windows PowerShell
    if (!$PSVersionTable.PSEdition -or $PSVersionTable.PSEdition -eq 'Desktop') {
        $ExpectedModulePathA = [Environment]::GetFolderPath('MyDocuments') + "\WindowsPowerShell\Modules"
        $ExpectedModulePathB = "C:\Windows\system32\WindowsPowerShell\v1.0\Modules"

        $PathSep = ';'
    }

    # Handles PowerShell Core on Windows
    if ($PSVersionTable.PSEdition -eq 'Core' -and $PSVersionTable.Platform -eq 'Win32NT') {
        $ExpectedModulePathA = "C:\Program Files\PowerShell\Modules"
        $ExpectedModulePathB = $(Get-Command pwsh).Source -replace 'pwsh\.exe','Modules'
        $ExpectedModulePathC = [Environment]::GetFolderPath('MyDocuments') + "\PowerShell\Modules"
        
        $PathSep = ';'
    }

    # Handles PowerShell Core on Linux
    if ($IsLinux) {
        $ExpectedModulePathA = "$HOME/.local/share/powershell/Modules"
        $ExpectedModulePathB = $(Get-Command pwsh).Source -replace 'pwsh','Modules'
        $ExpectedModulePathC = '/usr/local/share/powershell/Modules'

        $PathSep = ':'
    }

    # Handles PowerShell Core on Mac
    if ($IsMacOS) {
        $ExpectedModulePathA = $(Get-Command pwsh).Source -replace 'pwsh','Modules'

        $PathSep = ':'
    }

    $null = $ExpectedPaths.Add($ExpectedModulePathA)
    $null = $ExpectedPaths.Add($ExpectedModulePathB)
    $null = $ExpectedPaths.Add($ExpectedModulePathC)

    # Since we potentially added some $null values above, we need to filter them out...
    $UpdatedExpectedPaths = @($ExpectedPaths | Where-Object {$_ -ne $null})

    # Actually update $env:PSModulePath if necessary
    $UpdatedExpectedPaths | foreach {
        $ArrayOfPaths = $env:PSModulePath -split $PathSep
        $PathExistsCheck = $ArrayOfPaths -contains $_ -or $ArrayOfPaths -contains $($_ + '\') -or $ArrayOfPaths -contains $($_ + '/')

        if (!$PathExistsCheck) {
            $env:PSModulePath = $_ + $PathSep + $env:PSModulePath
        }
        if (!$(Test-Path $_)) {
            $null = New-Item -ItemType Directory $_ -Force
        }
    }
}