Test --ignore-dependencies behavior

@indirect noted that the bundler tests are slower because
--ignore-dependencies seems to be resolving dependencies.  This test
tests the existing behavior so a proper fix can be implemented.
Teach the resolver to ignore dependencies

Previously ignoring dependencies only occurred at install time, the
resolver still performed resolution on dependencies that would be
installed.

This made RubyGems much slower than 2.0.x which did not use the
resolver.

Now the resolver knows to ignore the dependencies during resolution,
along with RequestSet.

Thanks to @indirect for help discovering this problem.