bpdoc improvements

1. Extract module type documentation.
2. Support primary builder customization of factory function to use for
documentation for each module type.
3. Change the ModuleType list order so that they are grouped by package.

This is basically minor refactoring + readability improvement done on
top of https://github.com/google/blueprint/pull/232.

Change-Id: If7413e5ac23486b85f18d02fb3ba288a38730c32