Entry points don't work in reactive K8s operator charms

[johnsca]

Reactive K8s charms currently don't work due to the way layer:caas-base manages dependencies and builds the "venv". This layer needs to be updated to:

• Include the combined wheelhouse.txt processing that was added to charm-tools
• Construct the "venv" in the same way as charmcraft such that the package metadata is preserved so that entry points work

[johnsca]

Note: I consider this a somewhat low priority bug, since K8s operator charms are overall fairly new and should generally be written in the Operator framework. Additionally, K8s operator charms in general tend to be fairly straightforward to convert to the Operator framework. However, part of that conversion process is converting any interface protocol API libraries and being able to do that step separately makes it much easier to do the conversion.