go/types: improve the positioning of broken import errors

The heuristic gopls uses to guess error spans can get tripped-up on
certain valid characters in an import path (for example '-').

Update the error for broken imports to capture the full import path
span, so that gopls doesn't need to rely on heuristics.

Change-Id: Ieb8e0dce11933643f701b32271ff5f3477fecaaa
Reviewed-on: https://go-review.googlesource.com/c/go/+/300169


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>