cmd/gofmt: sort more, remove some duplicate imports

cmd/gofmt: sort more, remove some duplicate imports

* Sort imports by import path, then import name, then comment. Currently, gofmt sorts only by import path.
* If two imports have the same import path and import name, and one of them has no comment, remove the import with no comment. (See the discussion at issue 4414.)

Based on @rsc's https://golang.org/cl/7231070/

Fixes #4414.

R=gri, rsc
CC=golang-dev
https://golang.org/cl/12837044