Resolves PackageManagement builds on FullCLR and CoreCLR - #962

Darek Grala (@dgrala) I rebased your branch onto andschwa/package-management-files after rebasing it onto master. This should now be up-to-date with the latest sources.

I also amended your last commit so that it doesn't erroneously update the Pester module.

And pulled in Microsoft.PackageManagement/project.json as it originally wasn't added in a separate commit from the Source Depot import. <-- my bad rebase

Now unfortunately, I think .NET CLI's Windows builds just broke on us. Give me a minute.

I pinned the version of .NET CLI in #963 and reported it upstream; it shouldn't have broken like that.

I also need to fix up your project.json dependencies; there are a lot of extras.

Darek Grala (@dgrala) The projects restore and and the code is here... but now the question is, what is the top-level PackageManagement project that the host should take a dependency on so that this gets built?

Closing with #991.