Accept already extracted source-build-reference-packages output, not only a tar.gz

[dagood]

Source-build bootstrapping currently requires a tar.gz of `nupkgs produced by source-build-reference-packages. This means that there will necessarily be duplication on disk: a compressed and uncompressed copy. Distro maintainers can in some cases supply a pre-extracted directory (without significant extra effort), eliminating this data duplication. Source-build should allow it.

[dagood]

Already done in #1434. It looks like the API is --with-ref-packages <dir> for the scripts, driving /p:CustomReferencePackagesPath=... /p:SkipDownloadingReferencePackages=true to msbuild.