cmd/go: replace PackageInternal.GoFiles, AllGoFiles with methods

cmd/go: replace PackageInternal.GoFiles, AllGoFiles with methods

These are rarely used and can be computed on demand,
to make clear that they are never out of sync with the
lists in the non-internal Package fields.

Change-Id: I8c621dceaff1aeb39a3ed83f18e848adf14d7106
Reviewed-on: https://go-review.googlesource.com/56284
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>