Init consolidation

Fixes #381 (closed)

Repo has been renamed to LocalRepo to distinguish it better from BaseRepo (the superclass) and RemoteRepo.

Indexing is now done per-repository rather than per-prefix, and prefixes share index data based on the repositories enabled.

Prefix initialization logic has been moved out of portmod/_cli/init.py and into the add_prefix function, leaving the CLI to only prompt the user for the optional parts and choices. Similarly, The sync function in portmod/sync.py now does more work (can take BaseRepo instead of just LocalRepo, and will add repositories if missing) and less work is done in portmod/_cli/sync.py

Init will run a world update and introduce the user to the concept.