Fix detection of editable installation

When installing kolla-ansible with `pip install ./kolla-ansible`, pip
always creates a direct_url.json file, even when not using an editable
installation.

We see this behaviour with Python 3.12, while direct_url.json is only
created for editable installations on Python 3.9, which was used when
this code was initially developed for Kayobe.

When using a regular (non-editable) installation, this would make
kolla-ansible invoke site.yml from the source directory instead of the
virtualenv installation, causing a failure to load Ansible collections:

    Invalid plugin FQCN (ansible.utils.ipaddr): unable to locate collection ansible.utils

Fix by returning the source URL only if dir_info.editable is True.

Change-Id: Icdc2cedaa6a6e3a6b4351b1f4369e2e8b3a2dc97