Proper warning when the pyproject.toml options do not match existing options

[Pierre-Sassoulas]

Current problem

We're not going to crash following the merging of #4720 but we need to define better what is expected in the pyproject.toml for pylint. When using:

[tool.pylint]
load-plugins = []
disable = "logging-not-lazy,logging-format-interpolation"

load-plugins or disable are not valid options in the main namespace.

The expected configuration would be:

[tool.pylint]
    [tool.pylint."messages control"]
        disable = "logging-not-lazy,logging-format-interpolation"
    [tool.pylint."master"]
        load-plugins = []

But we're not warning the user about it.

Desired solution

We need to have proper error message when something is misplaced, because right now it's not very convenient. We're not going to crash following the merging of #4720 but we also should have an error message like:

disable is not a valid section for pylint, did you mean '[tool.pylint."messages control"] disable = ...' ?

Additional context

#4720 (comment)

[Pierre-Sassoulas]

Alternatively removing the sub-namespace could be more convenient when defining the configuration and also practical in pylint because each checker would have access to all options. and parsing would be a lot simpler. Maybe we could wait for the option/argparse refactor to do this.