Checklist

• I added a descriptive title
• I searched open requests and couldn't find a duplicate

What is the idea?

This work will be a continuation of the virtual package plugin hook implementation for conda.

What should happen?

Virtual packages defined by conda can be implemented as core plugins under the conda.plugins.* namespace and registered directly in conda.base.context.get_plugin_manager instead of via entrypoints.

Additional Context

This work is a continuation of the initial plugin implementation work and depends on #11811.