This PR optimizes the download tasks to reduce build times. After the change we see a order of magnitude reduction in download times for Windows package signing and also the compliance tasks.
In addition to that the PR also makes changes to organize the build artifacts in folders.
As of #7892, the PowerShell repository no longer uses Git submodules.
This is fantastic from a workflow standpoint, and so all the notes about
how to deal with submodules (and all the build steps explicitly
initializing and updating submodules) can be safely removed.
The main purpose of this was to enable full symbols for windows release build.
Also makes explicit where we are optimizing and where we are not optimizing due to https://github.com/dotnet/corefx/issues/29700
create a PowerShellPackage...ps1 that works with these VMs (vs our custom VM)
create a script to create the \powershell folder with correct permissions
a YML VSTS build definition
