Adds a new module `modules` with two helper functions for module
extensions:
* `use_all_repos` makes it easy to return an appropriate
`extension_metadata` from a module extension (if supported) to
indicate that all repositories generated by the extension should be
imported via `use_repo`.
* `as_extension` turns a WORKSPACE macro into a module extension that
uses `use_all_repos` to automate the generation of `use_repo` calls.
Alexandre Rostovtsev