always prune unused packages

[peterbourgon]

This is a question from my first run with the tool. I imported package A, which lives in repo R. Repo R also contains package B, which I don't use. Should/could dep prune B?

[jessfraz]

yes my opinion is it should

[jessfraz]

this is how gvt currently works

[sdboyer]

definitely, yes.

pruning gets all mixed up with checksumming in my mind, but i think pruning we can tackle first, which can then guide our approach to checksumming.

the big questions with pruning are, basically, "what's safe to prune." i'd really, really rather not provide any configuration controls around pruning - at most, there'd be an option to -skip-prune. but that means our pruning approach has to be pretty safe. it's easy to know which Go packages we don't need, but it's much harder to know whether pruning non-Go files, or directories without any Go files at all, is safe. maybe, in order to be safe, we just skip those entirely.

[sdboyer]

this expanded on in #120

[sdboyer]

Closing this in favor of #120