Moving packages to pkg/*

As discussed on https://github.com/cockroachdb/cockroach/issues/4182, if we want to keep non-cockroach written code in the repo (including, for example, a /vendor dir) we’ll want all the packages we write and maintain to live in a single, common subdirectory, and we settled on pkg for that.

I’m working on https://github.com/cockroachdb/cockroach/pull/9844 right now to do that, but any outstanding PRs might need a rebase after that lands – it likely won’t be too bad unless you changed or renamed a file (if you did, just make sure to rm the old location when you rebase)