Add documentation to Source#fetch_spec

Fix argument name for fetch_spec
Add Source::Git#load_spec

This loads the spec without creating a checkout in case resolution fails
and the gem cannot be used, there is a version mismatch, etc.
Implement prefetching from GitSet

This does not yet implement filtering
Add Gem::DependencyResolver::RequirementList#each

This will allow filtering during prefetching of git specs from a git
set.
Implement filtering for prefetched specs
Integrate GitSet#prefetch with find_all