Keep tags on selected deps so we know the context of their inclusion

[fabulous-gopher]

From @sdboyer on June 24, 2016 1:11

We need to be able to keep track of the context in which a dep is included, so that we can write that out into the lock file.

"Context," as in, "did this arise from a test dependency, and ONLY a test dependency?" Tracking test-only deps is the immediate use case for glide, but this should be the gateway to being able to track arbitrary tag contexts. And that's how we get OS/arch.

Copied from original issue: sdboyer/gps#44