Fix how directory contributions are loaded

This problem is currently visible only when overriding the userDirectory (using LDAPDirectory or MultiDirectory)

Let's say you have defined a new userDirectory based on a LDAP directory which overrides the default SQL directory.
Then if any other contribution registers a contribution to SQLDirectoryFactory, the order of the factories related to the userDirectory will be lost because the flush initiated by SQLDirectoryFactory will put SQLDirectoryFactory on top of the factories for userDirectory.

We need to have a mechanism which keeps the order of the factories for a given directory.