replay: Robustify external path removal implementation
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 #3870 (closed)
Fixes SWH-LOADER-SVN-53, SWH-LOADER-SVN-55 and SWH-LOADER-SVN-57.
Depends on !105 (closed)
Migrated from D7017 (view on Phabricator)