Add subpackages?

[earthboundkid]

In the course of working on #78, testing made it seem like Go's treeshaker isn't quite aggressive enough to shake out an unused init() from packages you don't use. So maybe we should move x/net/html into a reqhtml subpackage and TestConfig into reqtest.

The reqtest package could also have an always error transport and some other test mocking stuff. It could be really aggressive and also have the Replay and Caching transports in it.

I dunno. It's worth thinking about.

[earthboundkid]

Hmm, this could be a reason to do a breaking change to the repo URL.

[earthboundkid]

I think if I move to v2 at the same time, people won't get a broken upgrade message?

[earthboundkid]

Done #115