Fix ESLint 6 support

[ianschmitz]

Fixes #7510.

extends is no longer available in ESLint 6.

[mrmckeb]

One question, if that's OK - good to merge! Thanks @ianschmitz

[mrmckeb]

This threw an error previously if a file wasn't found. I understand it doesn't now?

[ianschmitz]

I think I may have been getting lucky (eslint config somewhere up the file system tree perhaps?). I was seeing errors in CI so i've reverted to include the catch again.

I also observed a strange error on our PNP test that failed due to eslint-config-react-app not being defined in package.json. This catch block is catching that error on master it appears.

[n1ru4l]

Please don't swallow the eslint config error. I think it would be better to throw the error incase the EXTEND_ESLINT environment variable is set.

E.g. I had some issues where a plugin referenced in the eslint config was not installed. I had to manually edit this file in order to print the error...

I would prefer a solution similar to this:

#7510 (comment)

[mAAdhaTTah]

You could also solve @n1ru4l's issue by only looking for a config file if EXTEND_ESLINT is set in the first place. Otherwise, it does a search for the file always but ignores it if the env var isn't set, which is a minor perf issue if you're not extending the config.

The other issue with this PR is the original wanted to ensure the react-app config was included, which this no longer does. Personally, I think if you're extending the ESLint config, you know what you're doing, and this should be allowed, but that doesn't seem to be in line with the project's philosophy.

[avindra]

This is great work all around. Thanks everyone. I sort of wish EXTEND_ESLINT would also disable the linting at build time (so that linting be handled separately (or in a different order) in people's build systems, for example). Is there any hope of such separation for create-react-app?