v2: vendor-neutral package names

Since moving to v2 (#661) allows for breaking changes, I want to make the proposal to rename the packages from "redis" / "redisx" to something vendor-neutral in v2, in the light of the whole "Redis relicensing + multiple forks emerging" situation.

I think, since this library is primarily a RESP client that doesn't try to mirror a particular database's command set in Go code, such a name change would reflect that approach quite well.

One option could be to simply move the "redis" subpackage's contents to the top level and to rename "redisx" to "experimental".