cmd/cover: use golang.org/x/tools/cover directly

cmd/cover: use golang.org/x/tools/cover directly

As suggested by Bryan in CL 249759, remove the forwarding aliases in
cmd/cover and use the symbols from golang.org/x/tools directly.

cmd/cover is not an importable package, so it is fine to remove these
exported symbols.

Change-Id: I887c5e9349f2dbe4c90be57f708412b844e18081
Reviewed-on: https://go-review.googlesource.com/c/go/+/304690
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>