Invest in improvements to the SDK repo

Copied and separating from https://github.com/dotnet/sdk/issues/42167
- Cleanup of old and new issues, test failures, and source
- Invest in improving existing tests
- https://github.com/dotnet/sdk/issues?q=is%3Aissue%20state%3Aopen%20label%3A%22Known%20Build%20Error%22
- https://github.com/dotnet/sdk/issues?q=is%3Aissue%20state%3Aopen%20label%3A%22Test%20Debt%22
- After we do a pass over the ones I've linked, we should consider just deleting all remaining skipped tests...
- Identify key customers bugs that need investigation and resolution
- Identify code for cleanup, refactoring, commenting, and documentation
- Identify code duplication that can be simplified (shared source files)
- Improvements to the SDK repo infrastructure
- ~~Combine redist and redist-installer~~
- Switch tests to use the installer output and reduce the overlaysdk impact
- Investigate and improve performance of dev, PR, and CI builds
- Investigate onboarding additional tests to helix (templating, format)
- Investigate improvements to the templating tests that are easily broken by live template changes
- ~~Enable Nullable for the SDK repo~~ and gradually enable for various projects
- Enable package source mapping for the repo and tests