[repository] Drop deprecated extid2eid API and friends This will break cwxmlparser based sources. They should be rewritten using a specific parser, based on xml representation or on rqlio. This is harsh but allows a so big cleanup of the code base. Furthermore, it's necessary for asource/extid handling in the entities table which is costly for most app that don't care at all about that... In this cset, delete: * all extid2eid methods * repo._extid_cache handling * [before/after]_entity_insertion source callback * the cwxmlparser and update related tests, notably unittest_datafeed where 'repull' testing has been removed, since it's now handled by the dataimport API and should not be retested there. Related to #15538288 Closes #15538383

### Requirements for tests in various cubicweb/**/test directories. ###

## shared by several test folders
cubicweb-card
docutils
Twisted < 16.0.0
webtest

## cubicweb/test
Pygments
mock
#fyzz XXX pip install fails
cubicweb-file
cubicweb-localperms
cubicweb-tag

## cubicweb/devtools/test
flake8

## cubicweb/hooks/test
psycopg2

## cubicweb/pyramid/test
pyramid >= 1.5.0
waitress >= 0.8.9
wsgicors >= 0.3
pyramid_multiauth

## cubicweb/sobject/test
cubicweb-comment