Remove the assembly version plumbing as it's no longer needed by our
build.

Change Microbuild to no longer be forced to use the MSBuild that
Microbuild provides. Doing so means we rely on them to update their
machines when we need updates to MSBuild.

Instead just use our normal mechanisms for discovering MSBuild. This
doesn't depend on machine state.

Our long term goal with asset publishing is to make it a completely
separate step from build. It's an operation which can optional be done
after builds complete. Even being done multiple times if needed to say
work around errors.

Additionally publish should be a deliberate, explicit operation. The
simple act of branching should not publish by default, it should be
explicitly opted into.

This moves us in that direction by doing the following:

- Centralizes our publish story to a single file: PublishData.json in
master
- Makes publish a step that can easily be run on any completed build
- Makes NuGet / VSIX publishing per branch and configurable. Allows for
uploads to many feeds if necessary
- Begins using dotnet/versions to track our packages.

This change moves all of the logic in Build.proj to microbuild.ps1. This
aligns with the rest of our infra changes that put scripting logic in
powershell vs. complex MSBuild files.

Correct the reference to the deleted BuildAndTest.proj file to use the build.ps1 script.

Our Test.cmd script and Microbuild test running got messed up during the
recent SDK change.  A series of conflicts were just merged incorrectly.

The Microbuild leg now almost completely builds.  It's failing in the
validation of our compilers Nuget package.  This comes from subtle
changes to Toolset.csproj that need to be addressed.

This does represent a huge step forward though.  Saving this now so I
can experiment with that change.

https://github.com/dotnet/roslyn/blob/master/docs/contributing/Powershell%20Guidelines.md

This makes two changes:

1. Move us to GitLink 3.0
1. Add source link info to all binaries we sign

Ensure that all *.*proj, *.props, and *.targets files are prefixed with the xml and copyright elements

This adds source server support to our binaries by means of GitLink.