Expand getLoader method to iterate over rule.loaders and rule.loader when applicable

[unconfident]

Why?

rule.loaders

From documentation:

This option is deprecated in favor of Rule.use.
Rule.loaders is an alias to Rule.use.

rule.loader

For starters, it's still supported by webpack despite what documentation says.

And it's a common, widely present situation in projects depending on CRA. Because whoever ported CRA's configuration to Webpack 2 didn't notice that .extract() method from ExtractTextPlugin returns an array. And so this array still being passed under loader key.

As result postcss-loader cannot be found with getLoader utility method of react-app-rewired in production configuration.

One way to match with webpack's logic is to depend on Webpack's very own internal method RuleSet#normalizeRules and, well, normalize rule set before performing the search. But this has two obvious downsides:

• It's an internal API and may change in the future
• It will throw errors if invalid configuration object will be provided

[unconfident]

Webpack's very own internal method RuleSet#normalizeRules

I was wrong, instance of webpack's RuleSet class isn't very suitable for needs of this project.

[timarney]

Closing this will track under the 2.0 discussion