Scheme fixes

This branch fixes a bug in the ReferencedContainer attribute generation in Scheme and adds support for aggregate build targets in the scheme generation. (Before, it would raise an error when generating hidden schemes for projects that have aggregate targets)

This will probably have a merge conflict in CHANGELOG.md if merged after my reference_for_path branch. I don't know what the nicest way to do this in GitHub is, this was the best way I could come up with.