mv cmd/new5l cmd/5l and so on.

Minimal changes to cmd/dist and cmd/go to keep things building.
More can be deleted in followup CLs.

Change-Id: I1449eca7654ce2580d1f413a56dc4a75f3d4618b
Reviewed-on: https://go-review.googlesource.com/6361Reviewed-by: Rob Pike <r@golang.org>