use relativize path from phpcr-utils

fix https://github.com/jackalope/jackalope-doctrine-dbal/commit/c203779fc8387150bc1fdfee3094705339f04317#commitcomment-7436081

needs https://github.com/phpcr/phpcr-utils/pull/123 to be merged first
not strictly related, but i wanted to figure out what this actually does, and then realized we don't need recursion to do that.