Allow `pwsh` to inherit `$env:PSModulePath` and enable `powershell.exe` to start correctly #11057

SteveL-MSFT · 2019-11-13T00:50:24Z

PR Summary

Since $env:PSModulePath is shared across all instances of PowerShell, PowerShell 7 needs to ensure it inherits the env var, but put its paths in front. If starting Windows PowerShell, it will special case powershell.exe and powershell_ise.exe removing PowerShell 7 specific paths creating a new environment for the new process.

This is a breaking change as previously, if pwsh detects it is started from PowerShell, it will clear out $env:PSModulePath and create it from scratch. This change will preserve additions from the system or user env vars but not all have the complicated logic in Windows PowerShell. It will always add user module path, shared module path, and $PSHome module path if it's not already there on startup.