When removing the subpaths of an external path, we first need to check
if a subpath is not versioned instead of checking if it is empty.
If a subpath is versioned, we can stop the paths removal process.
Relate to T3870
Fixes [SWH-LOADER-SVN-53](https://sentry.softwareheritage.org/share/issue/d6330b7c048249d99342e4c9d56dce19/), [SWH-LOADER-SVN-55](https://sentry.softwareheritage.org/share/issue/8c2e9f9bd1434295b75c3d5c83352f60/) and [SWH-LOADER-SVN-57](https://sentry.softwareheritage.org/share/issue/1a1463c747dd468087a08abc434c747d/).
Depends on D7015