[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.
HTTP cache management
=====================
.. automodule:: cubicweb.web.httpcache
Cache policies
--------------
.. autoclass:: cubicweb.web.httpcache.NoHTTPCacheManager
.. autoclass:: cubicweb.web.httpcache.MaxAgeHTTPCacheManager
.. autoclass:: cubicweb.web.httpcache.EtagHTTPCacheManager
.. autoclass:: cubicweb.web.httpcache.EntityHTTPCacheManager
Exception
---------
.. autoexception:: cubicweb.web.httpcache.NoEtag
Helper functions
----------------
.. autofunction:: cubicweb.web.httpcache.set_http_cache_headers
.. NOT YET AVAILABLE IN STABLE autofunction:: cubicweb.web.httpcache.lastmodified