Pub Tidy

lib/
    src/
        a/
            sub/
            sub.dart # generated with `export` files under `src/sub/`
        b/
    a.dart # generated with `export` files under `src/a/` 
    b.dart # generated with `export` files under `src/b/`

• path starts with _ as private will be excluded.
• only generate entry file for the first level under lib/.