gem_github is not part of the gem.deps.rb API
Complete GemDependencyAPI documentation

But the coverage of what the methods do is terrible
Remove comment for Gem::Util::popen
Complete documentation of Gem::Source
Add Source::Git#specs

This loads all of the specs in a git repository, not just the named one.
Fix Gem::Source#pretty_print with api_uri
Prefetch all specs in a git repository

A git repository can contain multiple gems, so prefetch all the specs,
not just a few to match bundler behavior.
Use GitSet#prefetch from #find_all for code reuse
Cache the GitSet#prefetch

We will call this multiple times via find_all, so we shouldn't be
repeating the work.
Remove Source::Git and Resolver::GitSet load_spec

These methods were used for loading a single gemspec from a git
repository, but this was wrong.  Now Source::Git#specs is used.