[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.