rename and deprecate top-level modules #640

These modules should not really be at the top level, they conflict with other people's packages (main one being ldif which is a big source of confusion), add a stub that reexports the same names and have imports trigger a Warning of some sort saying the stubs are going away in 5.0:

• _ldap -> ldap._ldap (rename+stub already happening in 3.5)
• ldif -> ldap.ldif
• ldapurl -> ldap.url

And update the documentation to match.