cmd/go: clear GOPATH from build context when importing from module

cmd/go: clear GOPATH from build context when importing from module

In module mode, we shouldn't handle packages under GOPATH any
differently from other packages. Clear GOPATH from the build context
before Importing to ensure that.

Fixes #37015.

Change-Id: I0203e25013716bca346fd4a67d80f1d05bbaea77
Reviewed-on: https://go-review.googlesource.com/c/go/+/412476
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>