gc: add -p flag to catch import cycles earlier

The linker would catch them if gc succeeded,
but too often the cycle manifests as making the
current package and the imported copy of itself
appear as different packages, which result in
type signature mismatches that confuse users.

As a crutch, add the -p flag to say 'if you see an
import of this package, give up early'.  Results in
messages like (during gotest in sort):

export_test.go:7: import "sort" while compiling that package (import cycle)
export_test.go:7: import "container/heap": package depends on "sort" (import cycle)

Fixes #2042.

R=ken
CC=bradfitz, dsymonds, golang-dev
https://golang.org/cl/4972057