Validation: Add support for overrides

The ESLint config validation script doesn't support overrides in our exported configs. If you add an `override` for a file pattern, the validator erroneously flags all top-level rules in the exported config as redundantly configured. We don't currently use overrides, but this should be fixed nevertheless.

Start with the ESLint config validation script and inspect how it processes exported configs containing an override for a file pattern. Reproduce the redundant-rule warnings, then verify that top-level rules are no longer falsely flagged when overrides are present.

Written by the indexing model from the issue text.