Did not consider these fields being embedded or adopted
into structs defined in other packages, but that's possible too.
Refine the import path check to account for that.
Fixes 'go test -short golang.org/x/crypto/ssh' but also
adds a new test in internal/boring for the same problem.
Change-Id: Ied2d04fe2b0ac3b0a34f07bc8dfc50fc203abb9f
Reviewed-on: https://go-review.googlesource.com/62152 Reviewed-by: Ian Lance Taylor <iant@golang.org> Reviewed-by: Adam Langley <agl@golang.org>