[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

# Check the CW versions and add the entity only if needed ?
add_entity_type('CWSession')
rql('DELETE CWProperty X WHERE X pkey "system.version.pyramid"',
    ask_confirm=False)

sql('DROP TABLE moved_entities')
sql('ALTER TABLE entities DROP COLUMN asource')
sql('ALTER TABLE entities DROP COLUMN extid')