Depending on zc.lockfile screws up PyPM buildout because the global
site-packages (of APy w/ PyPM) would already contain a zc/ namespace.

Hence, we avoid distributing namespace packages in APy.

Yet, we rely on zc.lockfile both on client and server. Instead of researching
on alternate locking modules and then verifying that everything works as usual,
we are settling in the shortest path .. which is to include zc.lockfile module
in pypm.common package.
