This fix, plus a one-line change to golang.org/x/tools/go/loader,
is sufficient to let that loader package process source code
using vendored packages. For example,

	GOPATH="" ssadump net/http # uses vendored http2

used to fail, not able to find net/http's import of the vendored
copy of golang.org/x/net/http2/hpack.

This CL plus the fix to loader (CL 17727) suffices to get ssadump working,
as well as - I expect - most other source code processing built
on golang.org/x/tools/go/loader.

Fixes #12278.

Change-Id: I83715e757419171159f67d49bb453636afdd91f0
Reviewed-on: https://go-review.googlesource.com/17726Reviewed-by: Ian Lance Taylor <iant@golang.org>