written by hand — uses an internal package from another module
(https://golang.org/s/go14internal)

-- .mod --
module golang.org/x/useinternal
-- .info --
{"Version":"v0.1.0","Name":"","Short":"","Time":"2018-07-25T17:24:00Z"}
-- go.mod --
module golang.org/x/useinternal
-- useinternal.go --
package useinternal

import _ "golang.org/x/internal/subtle"