Such errors include files without package decl, or conflicting package
decls. We ignore type errors, so we should ignore import errors too.
Fixes#10347
Change-Id: I9011a9099a2804281ea2d989d7263a9ce691be16
Reviewed-on: https://go-review.googlesource.com/8498
Reviewed-by: David Crawshaw <crawshaw@golang.org>