swift234

[docs] Update Modules User Model based on discussion and feedback.

- Drop the notion of hierarchical module names for now. People can prefix
  their module names (instead of their classes) until we come up with
  something else.
- For ambiguity resolution, prefer names in the current module (found by
  implicit visibility) over any sort of import (selective or otherwise).
- Add section on development-time Submodules based on last week's discussion.
- Simplify/remove several things that were found confusing.
- Still haven't written anything about search paths.

Swift SVN r9007