fix: stale HookRef in TaskProcessor, and cmf vs. bare compatibility

[tedhabeck]

Summary

This PR addresses a bug in isolated_venv 0.1.x and python compatibility with the CPEX 0.2.x framwork (#20)

Changes

• The TaskProcessor instances cached a HookRef which became stale after initialization. The PluginRef is now cached instead, and a fresh HookRef used on each execute_plugin call.
• For compatibility with invocation from CPEX 0.2.0, allow either cmf hook conventions or bare hook names.

Checks

• make lint passes
• make test passes