[pkg] separate leap requirements

[pkg] separate leap requirements

this is part of a process to make the setup of the development mode less
troublesome. from now on, setting up a virtualenv in pure development
mode will be as easy as telling pip to just install the external
dependencies::

  pip install -r pkg/requirements.pip

and traversing all the leap repos for the needed leap dependencies doing::

  python setup.py develop

- Related: #7288