config: add essential_plugins list, split out of default_plugins

[blueyed]

Agree about having two lists, and while at it, we should also stop running with a clear message if one of the essential plugins is passed to -p no:, "this internal plugin is essential for pytest function and cannot be blocked".

I would put those plugins in a list essential_plugins, and just make default_plugins a sum of the essential plugins with the rest:

essential_plugins = (
    "fixtures",
    "helpconfig",  # Provides -p.
    ...
)

default_plugins = essential_plugins + (    
    "debugging",
    "unittest", 
    ...
)    

But I believe this is better done in a separate PR, don't you agree?

Originally posted by @nicoddemus in #4965

[blueyed]

Done in #5005.