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

[sdboyer]

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.

[fabulous-gopher]

This issue was moved to golang/dep#445