Rely on cubicweb-skos's extid2eid mapping during LCSV data import
We drop monkeypatches and hacks introduced in 49d118aa8254 in 2016 which were supposed to be transient waiting for a cubicweb-skos release which came a long time ago now. Now we build the extid2eid dict before importing LCSV concept schemes and pass it to store_skos_extentities() so that it will be re-used for all files instead of re-built. Both lcsv_import and lcsv_check functions are updated, as the latter is used in test_dataimport.py as a kind of "mock" but must respect the former's interface. Also note that we now have to update 'extid2eid' dict after creating a new concept scheme (with init_seda_scheme()): while this sounds perfectly reasonable, I don't completely understand how this works without this before... We update requirement on cubicweb-skos so that the latest release is used (though some previous release would probably work as well, but I'm not sure which and it's not so easy to test).