//   - is a synthesized "main" package for a test binary, or
//   - contains only _test.go files.
func (p *Package) IsTestOnly() bool {
	return p.ForTest != "" ||
		p.Internal.TestmainGo != nil ||
		len(p.TestGoFiles)+len(p.XTestGoFiles) > 0 && len(p.GoFiles)+len(p.CgoFiles) == 0
}

type PackageInternal struct {
	// Unexported fields are not part of the public API.