fix: use baseConfig constructor option in FlatESLint

[mdjermanovic]

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Fixes #16341

What changes did you make? (Give an overview)

Fixed FlatESLint class to use baseConfig.

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

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[nzakas]

Overall LGTM. One scenario missing in the tests is when you are using both baseConfig and overrideConfig. It would be helpful to ensure that those are inserted in the correct order.

[nzakas]

Double checking: This may not be the best approach because basePath for a config file may not match the basePath that was passed in. Can you verify?

[mdjermanovic]

By basePath that was passed in, do you mean overrideConfigFile: <string> where the override config file is in another directory? We decided that in that case basePath should be cwd (#15683).

basePath is calculated and set here:

eslint/lib/eslint/flat-eslint.js

Lines 300 to 315 in c259ab7

	let configFilePath;
	let basePath = cwd;
	if (typeof configFile === "string") {
	debug(`Override config file path is ${configFile}`);
	configFilePath = path.resolve(cwd, configFile);
	} else if (configFile !== false) {
	debug("Searching for eslint.config.js");
	configFilePath = await findFlatConfigFile(cwd);
	if (!configFilePath) {
	throw new Error("Could not find config file.");
	}
	basePath = path.resolve(path.dirname(configFilePath));
	}

There are three cases:

• Normal eslint.config.js lookup (overrideConfigFile: false, default): basePath = the directory where eslint.config.js was found.
• Use an alternative config file (overrideConfigFile: <string>): basePath = cwd
• Don't use any config files (overrideConfigFile: true): basePath = cwd

[nzakas]

Got it. 👍 just wanted to double check.

[mdjermanovic]

One scenario missing in the tests is when you are using both baseConfig and overrideConfig. It would be helpful to ensure that those are inserted in the correct order.

Done. I've added a test with baseConfig + overrideConfig, and a test with baseConfig + config file + overrideConfig.