cmd/go: clean up adding import path to command error
Currently, cmdError makes a somewhat fuzzy attempt to ensure the
package import path is part of the printed error, using a string
prefix check. Also, if it decides it does need to add the import path,
it prints it as a "go build" line, which could be misleading because
it can happen outside of "go build".
Clean up the whole code path by explicitly checking the provided error
description against Package.Desc(), and instead of emitting "go build"
in the error message, print it as "# importPath" just like we do in
the common case.
Change-Id: Idb61ac8ffd6920a3d2d282697f4d7d5555ebae0c
Reviewed-on: https://go-review.googlesource.com/c/go/+/534655
Auto-Submit: Austin Clements <austin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Bryan Mills <bcmills@google.com>