Entry points don't work in reactive K8s operator charms

Reactive K8s charms currently don't work due to the way [`layer:caas-base`](https://github.com/juju-solutions/layer-caas-base) [manages dependencies and builds the "venv"](https://github.com/juju-solutions/layer-caas-base/blob/master/tactics/wheelhouse.py). This layer needs to be updated to:
* Include the [combined `wheelhouse.txt` processing](https://github.com/juju/charm-tools/pull/569) that was added to `charm-tools`
* Construct the "venv" in the [same way as `charmcraft`](https://github.com/canonical/charmcraft/blob/master/charmcraft/commands/build.py#L239-L280) such that the package metadata is preserved so that entry points work