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
You're right, I'll add it too to that fix.

Note that we "suffer" from the same issue today (on master). Can you think of a test case to reveal this issue?