Convert pvc-client-cli into a proper Python module

Also fixes up the Debian packaging such that this works how I would
want, with proper module installation while leaving everything else
untouched. Finally implements automatic installation and removal of the
BASH completion for the PVC command.

debian/pvc-client-cli.postinst

@@ -1,4 +1,6 @@
 #!/bin/sh
 
-# Install client binary to /usr/bin via symlink
-ln -s /usr/share/pvc/pvc.py /usr/bin/pvc
+# Generate the bash completion configuration
+if [ -d /etc/bash_completion.d ]; then
+    _PVC_COMPLETE=source_bash pvc > /etc/bash_completion.d/pvc
+fi