generalize package_dir to fix setuptools' develop

Running setuptools' develop target (as well as, by extension, `pip
install -e`) on an ansible checkout does not work.  This is a
limitation of the develop target, but it can be made to work if
install_dir is generalized to say that *all* packages are in lib.
(cf. https://github.com/pypa/pip/issues/126#issuecomment-874855)

Note that this does not address the modules in library.  Setting
the ANSIBLE_LIBRARY environment variable appropriately will, though.
These two items are sufficient to use a source checkout in a
virtualenv without using the env-setup script.