[repository] Drop the entities.extid column and associated cache This was not necessary anymore with promoted usage of the new data import API. Turn repository's _type_extid_cache to _type_cache with only the entity's type as key. This introduces an backward incompatible change: entity_metas dict doesn't contains anymore the extid key, but it doesn't seem used at all anywhere, so this sounds acceptable. Closes #15538317

.. _rset_module:

:mod:`cubicweb.rset`
====================

.. automodule:: cubicweb.rset

    .. autoclass:: ResultSet
        :members: