fix module inclusion path when running from source

The previous search paths assumed that ansible was installed on the
system. This meant that it did not work if running ansible from a
source checkout without installing it via pip (or other means.) More
insidiously, if a user was running from a source checkout, but also
installed ansible in system paths, ansible-shell used the system
version of ansible modules, not the source version.

Rebased to effectively undo Lorin Hochstein's commit cc47d4c. While
that commit also worked I think this version is more correct as it
ensures that ansible-shell module path behavior mirrors that of ansible
itself. ansible uses DEFAULT_MODULE_PATH, not DIST_MODULE_PATH to set
module_path if -M is not used, as shown on line 474 in
lib/ansible/utils/__init__.py