[dataimport] dispatch and deprecate old code previously, the whole dataimport.py content has been copied in dataimport/__init__.py, then dataimport/importer.py API has been introduced. Following these changes, this cset: * dispatch __init__ code into sub-modules * dispatch unittest_dataimport accordingly * deprecates functions and classes that are deprecated by the new API Related to #5414753.

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missing-dep-for-interpreter make => make | build-essential | dpkg-dev (usr/*/cubicweb/skeleton/debian/rules.tmpl)