cmd/vet: check that C receivers are cgo imports

cmd/vet: check that C receivers are cgo imports

Otherwise, vet might have false positives when "C" is a variable and
we're just using a method on it. Or when an import was renamed to "C".

Add test files for both of these cases.

Fixes #20655.

Change-Id: I55fb93119444a67fcf7891ad92653678cbd4670e
Reviewed-on: https://go-review.googlesource.com/45551
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Rob Pike <r@golang.org>