Bring `get_plugin_manager` inline to `conda.base.context`

[kenodegard]

Description

Since the get_plugin_manager isn't called directly, let's just fold the entire implementation into the context object.

Checklist - did you ...

• [x] ~Add a file to the news directory (using the template) for the next release's release notes?~
• [x] ~Add / update necessary tests?~
• [x] ~Add / update outdated documentation?~

[kenodegard]

@jezdez no idea, perhaps @beeankha knows?

an alternative question, since we now have the conda.plugins submodule can we just move this there?

[kenodegard]

@jezdez ah looks like it's origin is from the getting started docs:

[jezdez]

Let's close this since I don't think we want the plugin manager in the context anymore.