Update: config-validator should validate overrides

[mysticatea]

What is the purpose of this pull request? (put an "X" next to item)

[X] Bug fix

What changes did you make? (Give an overview)

This PR fixes a bug that ESLint overlooks invalid env and rules option values in the overrides option. This is important to catch invalid rule options.

This might be a breaking change since it can throw new error on invalid configs.

Is there anything you'd like reviewers to focus on?

Nothing in particular.

[platinumazure]

I think this should probably be labeled as a breaking change (and this needs to be discussed in a TSC meeting anyway), but otherwise this LGTM. Thanks!

[mysticatea]

TSC Summary: The ConfigValidator#validate overlooks invalid rules and env options in the overrides option. As a result, ESLint doesn't report config errors such as:

{
    "overrides": [{
        "files": "*",
        "rules": {
            "no-undef": ["error", { "InvalidOption": true }]
        }
    }]
}

This PR fixes the bug. This PR is marked as a breaking change since ESLint can throw new config errors on invalid configs.

TSC Question: Should we accept this PR?

[not-an-aardvark]

I think this is a useful change, but I'm hoping we don't have to delay the 5.0 release for it because we've already received two requests to backport changes.

[mysticatea]

I'm hoping we don't have to delay the 5.0 release for it

I agree.

[btmills]

Code LGTM, pending formal acceptance by the TSC.

[platinumazure]

Please feel free to dismiss my review if needed once TSC agrees to accept this. I only left the "request changes" so that an accidental merge would be harder to do.

[alberto]

It's purely semantics at this point, but IMO it's an enhancement. Config validator came before overrides, so we never agreed to validate overrides (which I think we should)

[alberto]

LGTM

[platinumazure]

Looks like we have "quorum" on this PR. With Ilya on vacation and Nicholas working on his health, it's really just @gyandeeps we're missing here.

@gyandeeps Any objections to this change?

Allow merge before release