Canonicalize github urls for the resolver

In addition to canonicalizing checkout locations, this canonicalizes packages
for the resolver. This allows two dependencies with slightly different urls
pointing to the same repository to resolve to the same location and package.

Closes #104
auto merge of #123 : alexcrichton/cargo/canonicalize-github-urls, r=wycats

In addition to canonicalizing checkout locations, this canonicalizes packages
for the resolver. This allows two dependencies with slightly different urls
pointing to the same repository to resolve to the same location and package.

Closes #104