feat: Introduce ldap.ldif and ldap.url - #658

I don't know why these constants are duplicated and exposed here.
But okay with adding the missing one until that is clarified.

Probably because the module was ensured to work without _ldap / libldap which is not possible anymore after this move.

Only realised we've promised that today. It's not been possible since the type hinting PR but more importantly, the promise to use ldif and ldapurl independently sort of failed since we never put them on pypi separately and now ldif exists there and is an incompatible implementation, we'll never be able to.

I avoid the rewrites that would be relevant if we formally dropped that promise at this point so we can try and fix it in 3.5.x if people show us that they care about the promise but we'll need to know how they manage to extract them when there is no official way to do so AFAIK.

Still happy to do sensible cleanups/tweaks that do not affect the above.

And double checking just now, ldif looks very much maintained but pinging their maintainer (according to pypi at least) @sfermigier so we don't have to guess on the above.

@sfermigier: Just to fill you in, we're not asking for anything (much less for you to give up the ldif name!), just wanted to know what your position was re: your ldif module and python-ldap's since the confusion must have affected you as well