go/build: add Package.IgnoredOtherFiles

Just like it is useful to report ignored .go files,
it is also useful to report ignored non-.go files.

Also, remove _* and .* files from IgnoredGoFiles.
The fact that they were there at all was a mistake.
This list is for files being ignored due to build constraints.

Change-Id: Ie08252dde9f56bda08647e7ebeab7906e9271c54
Reviewed-on: https://go-review.googlesource.com/c/go/+/240551


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>