[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

[repository] Drop the entities.asource column It not used anymore thanks to previous patches. Closes #15538288

[integrity check] Stop using entities.asource column in integrity checks Simply check if entities have a cw_source, and link it to the system source when not found. Related to #15538288

[rql2sql/test] Ensure test results predictability Without this, test were randomly failing within tox due to different order of solutions.

[rql2sql] Stop generating SQL query from RQL using entities.type This was necessary in the "true multi-sources" time, it's not anymore, while maintaining this index is costly. Related to #15538317

[test] Remove some usages of lgc.testlib

[test] Avoid pytest discovery warnings Having TestServerConfiguration and alike in the test module namespace causes pytest discovery errors, because it thinks it's a test class.

flake8 and all * update some copyright * drop most __docformat__ declaration * fix some flake8 warnings / errors

[schema2sql] Drop deprecated comment

[cnx] Use entity_type instead of entity_metas()['type'] The latter is deprecated in favor of the former which has been introduced on cnx to replace it.