Building off of #7286, we need a way to let users list plugins that are absolutely required. We should have pytest emit a warning when a required plugin is not found, and the --strict-config flag should turn these warnings into errors if passed

The key for this should be called required_plugins and it should be a comma separated list of plugins that must be installed

So it would look like:

[pytest]
required_plugins = pugin1,plugin2,...,pluginN