go/buildutil: changes for vendor support Add AllowVendor constant for convenience across x/tools. (Will delete go/loader's importMode later) Add StripVendor utility function + test. Change-Id: I885076cb4ea67588996d85b749b85f49cd619f0d Reviewed-on: https://go-review.googlesource.com/18049 Reviewed-by: Robert Griesemer <gri@golang.org>