Adjust multipip usage.

Use json as the output format and only use stderr for
when the multipip program really fails.

Instead of outputting to the screen all incompatible errors
just output the incompatible versions to the output json
data structure and use this in the main anvil program.

Use defaultdict + list in multipip where appropriate.

Ensure that as documented we select the first matching requirement
that comes out of multipip instead of the last (when incompat. matches
happen).

Adjust documentation for new json output format.

Change-Id: I6403182066ef01cb3f7f26c305ab5577a00043ae