misc/vim: fix Drop for imports after the first group.

misc/vim: fix Drop for imports after the first group.

Previously, an import block such as
	import (
        	"net"

                "stack"
        )
would not permit ":Drop stack" to work because we were aborting
the scan early, which is only correct when Import is in operation.

R=golang-dev, franciscossouza
CC=golang-dev
https://golang.org/cl/6532053