Rubygems sources are redundant

[sam]

Because Rubygems is a terrible library and uses globals all over the place, we set Gem::sources to assign the repository for the SpecFetcher and DependencyInstaller. We create a GemSource per-repository however. Which means we're actually scanning all the sources multiple times.

If you only have one source, this isn't an issue, but if you have more than one, it's just redundant work before performed.

We either need to figure out a better way to leverage the Rubygems lib, or we need to replace these two classes with our own implementation.