mod=example.com/main
base=v0.0.1
-- want --
example.com/main/b
------------------
Compatible changes:
- package added

Suggested version: v0.1.0
-- go.mod --
module example.com/main

go 1.14
-- a/a.go --
package main

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

func main() {}