[test] Drop an unused function in cwconfig tests

[cwconfig] Always use "sys.prefix" to determine installation prefix We drop all legacy logic implemented in _find_prefix() which now happens to be buggy since we cleaned up setup.py in 3.26 (noticeably because virtualenv is no longer properly detected). Closes #17132990.

[cwconfig] Look for migration scripts directory relatively to code Thus getting rid of the last piece of data in "shared" directory. As a consequence, we can drop data_files from __pkginfo__.py. Related to #17132990.

[pkg] Get rid of "pydir" in __pkginfo__.py Nothing uses this.

[pkg] Get rid of "shared" data These are not used anymore per previous changesets. Related to #17132990.

[cwconfig] Drop shared_dir method There's no more users of this method. Related to #17132990.

[config] Avoid relying on shared_dir method when possible From 3.26, some "data" directories (like web/data and i18n or wdoc) are installed along with CubicWeb in site packages. So we can retrieve them relatively to the module that needs them (namely "cubicweb" for "i18n" and "cubicweb.web" for "web/data" and "wdoc"). Reason for doing this is that "share_dir()" is not working when cubicweb is installed in "develop" mode and we are about to drop this method. Related to #17132990.

[test] Add a test for WebConfiguration.locate_all_files() We are going to change the implementation, make sure there's no regression.

[cwconfig] Get rid of _forced_mode global And compute CubicWebNoAppConfiguration.mode within the class namespace. Related to #17132990.

[cwconfig] Get rid of CWDEV This is in preparation of a fix for #17132990 to cleanup legacy (and probably no longer working) behaviors.