Remove satisfied dependecies from next source lookup

When iterating sources and checking for met dependencies, remove the
actual specs that were satisified by the source from the lookup of
following sources.

This prevents lookup of local gems (from `gemspec` or `path` or `git`
sources) in rubygems source.

fixes #2909
If a second source contains the same gem, I guess? So like two git sources that both contain the gem "foo". Bundler should warn about that, I think.