setup.py: add a read_module_contents() helper
setup.py: add a read_module_contents() helper

Prior to this change, we would read __init__.py without closing the
filehandle, because it was meant to be used only once.

Move this to a helper function so we can re-use it elsewhere.

ceph/ceph-installer,ceph/mariner-installer,ceph/ceph-installer,ceph/ceph-installer