Add the Arcade-powered source-build wrapper.

[crummel]

This adds a source-build project in eng/source-build. This project pulls in a recent version of Arcade that supports the ArPow targets, restores arcade tooling (so we make source-build tasks available), builds NuGet in the source-build fashion (Restore/Build/Pack XPlat), and then collects the built NuGet packages into source-build intermediate packages.
Remaining work:

• publishing the intermediate packages.
• restoring dependencies from source-build intermediate packages (this requires updates to eng/Version.Details.xml as well to make sure source-build knows what the real dependencies are).
  I wanted to get this in as an intermediate step because the dev workflow for source-build does work now.

There are a couple NuGet source changes I made:

• Added a global.json in the root and renamed NuGet.Config to NuGet.config (Arcade looks for these in the root and the paths can't be easily changed. I can try to find a workaround if this is an issue).
• Excluded trying to import some VS targets in source-build.

Bug

Fixes: dotnet/source-build#2069.

Regression? No. Last working version: N/A

Description

PR Checklist

• PR has a meaningful title

• PR has a linked issue.

• Described changes

• Tests

  • Automated tests added
  • OR
  • Test exception
  • OR
  • N/A
    This adds a source-build CI leg which is effectively the test of the source-build changes.

• Documentation

  • Documentation PR or issue filled
  • OR
  • N/A

[crummel]

@dominoFire Could I get the CI label on this please?

[crummel]

ping @nkolev92 @dominoFire for the CI label please.

[crummel]

@dominoFire I merged #3945 into this one so I closed that one. cc @dseefeld and @MichaelSimons if you'd like to review.

[crummel]

@dominoFire @nkolev92 @zivkan Is there any other changes that need to be made before you can merge this?

[zivkan]

@crummel Out of curiosity, why is NuGet.Config being renamed to NuGet.config? nuget.config is actually the preferred style on Linux, but all 3 are read.

Furthermore, this repo is mirrored to Azure DevOps' DevDiv instance, which has "Case enforcement" enabled, and therefore this change might break mirroring.

[crummel]

This is due to an Arcade issue. I've reverted this for now and put in a source-build patch that does it instead - I can remove this patch when we bring in intermediate package publishing to Nuget.

[zivkan]

Let's hope we can get a non-flaky CI build soon.

[crummel]

Thanks for all your help getting this in!