go/build: change the search order for "vendor/" paths based on srcDir

If srcDir is within GOROOT, prefer GOROOT.
Otherwise, prefer GOPATH.

The attached tests may seem a bit strange; they will make more sense
in a followup CL.

Updates #16333
Updates #30241
Updates #30228

Change-Id: Ic5f1334cce5e242d7f49080aba083bcf2080dee3
Reviewed-on: https://go-review.googlesource.com/c/go/+/164619


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>