go/build: reject directory with only cgo files if cgo not in use

The old test for "no Go files" was p.Name == "", meaning we never
saw a Go package statement. That test fails if there are cgo files
that we parsed (and recorded the package name) but then chose
not to use (because cgo is not available).

Test the actual file lists instead.

Fixes #6078.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/13661043