cmd/go: allow 'go run x.go' to use nearby internal imports in module mode

In GOPATH mode the rule has always been that 'go run x.go' can
import whatever the package in x.go's directory would be able to
import. Apply the same rule here.

The bad import path was triggering other mysterious errors
during 'go run' in other circumstances. Setting it correctly fixes
those too.

Fixes #26046.
Fixes #27022.

Change-Id: I0a9b0a154a20f48add5a199da85572e7ffe0cde4
Reviewed-on: https://go-review.googlesource.com/129798


Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>