slapproxy: Fix software URL migration

The local-software-release-url option allows migrating the software URLs which are local paths by rebasing theml on the path provided by the option. See https://lab.nexedi.com/nexedi/slapos.core/merge_requests/288

Do not migrate software release URLs if the old root path and the new root path are subpaths or superpaths one of the other.

In addition, do not migrate an URL if the old one refers to an existing file and the new one doesn't.