collections.inc: sync with MVL6.

collections.inc: sync with MVL6.

- Gather info on the collections into COLLECTIONSINFO.
- Store the collection info for the current file in COLLECTIONINFO.
- Add has_collection() and get_collection() utility functions.
- Make collection_unpack pass back the collection name.
- Just use != rather than symmetric_difference on the set comparision.
- Set PYTHONPATH for the bitbake re-execution.
- Make the 'Using existing' message use bb.debug.

Signed-off-by: Chris Larson <clarson@mvista.com>