(RE-1040) Support building gems on Windows

This commit makes it possible to execute the packaging repo task `package:gem` on Windows, and have it create both the generic and platform-specific Puppet gems. This supercedes https://github.com/puppetlabs/packaging/pull/259.

You will need `git` and `msys` in your path to account for things like `mktemp`
where does the be_windows matcher come from?