internal,cmd/internal: relocate covcmd package from std to cmd

internal,cmd/internal: relocate covcmd package from std to cmd

Relocate the 'covcmd' package from .../internal/coverage to
.../cmd/internal/cov, to reflect the fact that the definitions in this
package are used only in cmd, not in std.

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-windows-amd64-longtest
Change-Id: I65bcc34736d1d0a23134a6c91c17ff138cd45431
Reviewed-on: https://go-review.googlesource.com/c/go/+/526595
Reviewed-by: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>