Generators: look for generators in all gems, not just those suffixed with _generator, in the gem's generators or rails_generators directory. Allow use of the rails_generators directory instead of the standard generators directory in plugins also. Closes #8730.

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@7139 5ecf4fe2-1ee6-0310-87b1-e25e094e27de