File extension and path checks need to take into account case-sensitive file systems

https://github.com/PowerShell/PowerShell/search?utf8=%E2%9C%93&q=ordinalignorecase+getextension&type=Code

(query may not be complete)

On Linux/Mac, we should follow any precedent dotnet has already defined for assemblies (in case there's foo.dll and foo.DLL)
