[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.
../../doc/book usr/share/doc/cubicweb-documentation
../../doc/html usr/share/doc/cubicweb-documentation
../../debian/cubicweb-doc usr/share/doc-base/cubicweb-doc