cmd/go: check that package paths are invariantly non-empty
The empty string is never a valid package path.
Passing an empty string to a function that expects a package path
indicates some missing validation step further up the call chain —
typically (and most easily) a missed error check.
Change-Id: I78a2403d95b473bacb0d40814cd9d477ecfd5351
Reviewed-on: https://go-review.googlesource.com/c/140857
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org>