Make YastModule#{ybc_deps,ruby_deps,transitive_deps} return objects

This commit makes YastModule#{ybc_deps,ruby_deps,transitive_deps} return
YastModule objects instead of module names. This has several benefits:

  * Code using these methods is simpler as it doesn't need to look up
    the modules.

  * There are less references to |$yast_modules| scattered over the
    code (this is related to the previous point).

  * It will make topological sorting of YaST modules (something that
    will need to be implemented in order to make |yk compile all| run
    properly on a clean system) easier to do.
Merge pull request #235 from yast/deps-methods-return-objects

Make YastModule#{ybc_deps,ruby_deps,transitive_deps} return objects