example.com/cmd contains main packages.

-- .info --
{"Version":"v1.0.0-newerself"}
-- .mod --
module example.com/cmd

go 1.16

require example.com/cmd v1.0.0
-- go.mod --
module example.com/cmd

go 1.16

require example.com/cmd v1.0.0
-- a/a.go --
package main

func main() {}
-- b/b.go --
package main

func main() {}
-- err/err.go --
package err

var X = DoesNotCompile