MSBuild migration follow ups

[pranavkm]

• Consolidate bootstrapping of a project in to a bootstrapper project in KoreBuild. To build a repository, we have to be able to restore KoreBuild.proj in the context of the project's repo.props and then run build on it. This involves copying files around (might be unavoidable) and duplicating what KoreBuild.ps \ KoreBuild.sh does: https://github.com/aspnet/KoreBuild/blob/dev/build/KoreBuild.ps1#L116-L117. Would be nicer to do this via msbuild.
• Investigate using project references rather than package references as part of creating Coherent build. See Update to MSBuild #478 (comment)
• When running test builds, we do a regular build which includes building and verifying packages, running API checks, etc. We should investigate doing less work here.
• Make pointing to a directory of packages i.e. changing _DependencyPackagesDirectory a first class concept. We could use this as part of mirroring. (See https://github.com/aspnet/CoreCLR/pull/156/files#diff-e45ce6afc81adf6a66a1064a7eeb7ca1R35)

[pranavkm]

cc @natemcmaster