cmd/go: clarify that -coverpkg uses import paths
This change amends the long-form help output for 'go help build' and
'go help testflag' to specify that the '-coverpkg' flag operates
explicitly on import paths as well as package names. Import paths are
fundamental for precise specification of packages versus unqualified
package names, and the naming of the flag '-coverpkg' and its original
documentation leads a user to assume that it only operates on the
simple, unqualified package name form. The situation warrants
clarification.
Fixes #69653
Change-Id: Ifde6a974405ce1614e28898fc2b92ed5bad94e57
GitHub-Last-Rev:
466c662a70d13560f2fb7affb0d7d900889c7aa7
GitHub-Pull-Request: golang/go#69655
Reviewed-on: https://go-review.googlesource.com/c/go/+/616257
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>