Improve caching logic

[alexlafroscia]

It's pretty clear that there are some issues with the caching, namely that configuration changes don't invalidate the cache. A number of issues have come up with that as the root problem:

• ember-cli-eslint #11
• ember-cli-eslnit #118

I wanted to make an issue here as a reminder about this and a place to start gathering information for putting together a fix.

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ESLint does provide a way to programmatically get the ESLint configuration for a particular file, namely the getConfigForFile function. Maybe we can use this information as part of the caching mechanism? My concern would be that this would be a pretty slow approach, but I don't know how poor the performance would actually be. I do feel like this might be a good approach because it would take any changes to the configuration into account, whether it's a change to a config file or adding things to an eslintignore, and would be robust going forward as long as their Node API does not change.

[alexlafroscia]

Maybe I spoke too soon. It looks like we're already using that as part of the caching process...