With the new package layout (everything under "cubicweb" package), the custom
install_lib rule which makes use of include_dirs defined in __pkginfo__.py did
not prepend the package name to source directories to be copied. Fixing this.

Also, in setup.py's export() function, the destination directories' path to be
created during source tree walk was wrong.

All this makes cubicweb/skeleton directory (which is not a package) properly
installed by setup.py.

The test in cubicweb/devtools/test/unittest_devctl.py wasn't properly
implemented because it used an installation of cubicweb in "develop" mode
which shadows such packaging issues. Also it used "python -m cubicweb" instead
of directly "cubicweb-ctl" and the former appears to fall back to using the
cubicweb package *from sources* instead of the installed one.

Now that this test runs against the installed version of cubicweb, fix
MANIFEST.in to include tox.ini files (cubicweb's and skeleton's) as this is
expected from the test.

Closes #14127941.