cmd/cover: copy to standard repository from golang.org/x/tools/cmd/cover

This required dealing with the ill-advised split of the profile code
into a separate package. I just copied it over unchanged. The package
does not deserve to be in the standard repository. We can cope
with the duplication.

Also update the go command to know about the new location.

Fixes #10528.

Change-Id: I05170ef3663326d57b9c18888d01163acd9256b6
Reviewed-on: https://go-review.googlesource.com/9560


Reviewed-by: Russ Cox <rsc@golang.org>