Jennis/fix dpkg plugin

The dpkg plugin is breaking due to changes in the BuildStream's main codebase regarding our new policy regarding private symbols (https://buildstream.gitlab.io/buildstream/HACKING.html#policy-for-private-symbols)

Namely, the derived DpkgElement class cannot access the __commands variable of it's base (BuildElement) class.

There is a simple workaround for this that has been implemented in this patch. It should be noted that this workaround DOES NOT revert the former change of commands to __commands

Also, progression in the main BuildStream codebase meant that the runcli.py file we used in tests became deprecated. This has been replaced with the most recent version.