The build list is very incomplete in vendor mode,
so we can't rely on it in general.
findModule may be called in modload.PackageModuleInfo, which
load.LoadImport invokes relatively early during a build.

Before this change, the accompanying test failed at 'go build
-mod=vendor' with the message:

	build diamondpoint: cannot find module for path diamondpoint

Change-Id: I5e667d8e406872be703510afeb079f6cbfdbd3c8
Reviewed-on: https://go-review.googlesource.com/c/140861
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>