[devtools] Update skeleton's setup.py to install cube as a package Most of the prior logic of skeleton's setup.py gets dropped as installing a cube as a "classic package" (i.e. in site-packages) is just the default behavior of distutils. Also add a test checking installation of new cube. Related to #13001466.

[devtools] Adjust skeleton setup.py to have it read information from __pkginfo__.py Since the later is now installed in cubicweb_<CUBENAME> directory. Add a test ensuring that `python setup.py sdist` works on newly created cube (which should at least verify that setup.py is valid). Related to #13001466.

[devtools] Update skeleton and newcube command to cube as package layout Related to #13001466.

[pkg] Add missing entries to MANIFEST.in Detected by running `check-manifest <https://github.com/mgedmin/check-manifest>`_. Add a tox environment accordingly. Closes #15229018.

[test] Add a test for toolsutils.read_config

[test] Use plain unittest in unittest_toolsutils

[skeleton] Remove test/pytestconf.py

[pkg] Make it clearer which entries of __pkginfo__ are actually used

[test] Avoid usage of lgc.testlib TestCase in unittest_cwconfig.py

[test] Move setup of spa2rql tests into setUpClass method Thus avoiding cryptic errors upon unittest discovery.

[schema] Set CubicWebSchemaLoader's extrapath using config eponymous property

[doc] Document change of instances location in virtualenv installation Closes #14789440.