Support global Yarn installs and better errors

[Arcanemagus]

Add support for global Yarn installations. Apparently one of the few things Yarn did right is that all platforms use the same directory structure under the base directory, unfortunately that meant that on non-Windows platforms specifying the path still got you an unusable setup.

Also fixes the error messages that can be thrown to better reflect the actual causes of the messages.

Fixes #680.
Fixes #780.

[Arcanemagus]

Hmmm, the spec failure is due to the fact that all platforms now check the node_modules/eslint path first, so it's finding that path even on the non-Windows platforms.

Do we want to split that folder up, or just remove the lib variant?

[IanVS]

Looks good, and seems to work fine on my mac. I don't have a windows machine set up to test, but I'll trust you there and the code looks sane.

[Arcanemagus]

Released in v8.2.0 🎉.

[selfagency]

I'm on a Mac, I have 8.2.1, I have eslint installed via yarn, and I cannot get it to find eslint even though it's in the $PATH and works fine on the command line. My yarn global prefix is /usr/local and the symlink to the yarn node_modules folder is in the /usr/local/bin folder.

[Arcanemagus]

@selfagency The only thing that this package uses $PATH for is resolving npm when attempting to get the prefix automatically. Since you are using yarn you will need to specify it manually. Please see #780 for some locations where it might be.

Note that using a global ESLint install is very much discouraged, but supported, you should be using a local instance in your project instead!

If you are still having problems please file a new issue.