cmd/go: local import fixes

cmd/go: local import fixes

1) The -D argument should always be a pseudo-import path,
like _/Users/rsc/foo/bar, never a standard import path,
because we want local imports to always resolve to pseudo-paths.

2) Disallow local imports in non-local packages.  Otherwise
everything works but you get two copies of a package
(the real one and the "local" one) in your binary.

R=golang-dev, bradfitz, yiyu.jgl
CC=golang-dev
https://golang.org/cl/5787055