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>
5 files changed