Support new file and helper selection

[novemberborn]

avajs/ava#2103 changes how test files and helpers are selected. This impacts our no-ignored-test-files and no-import-test-files rules.

I suggest we add an eslint-helper file to the main ava package. It should take care of reading the config (accepting a project directory argument). The rules should then be able to call a function to determine whether something is a test or helper file.

There may be other places where we try to resolve AVA config. We should move those to ava/eslint-helper as well.

[sindresorhus]

@novemberborn Was this fixed by df6374a?

[novemberborn]

Yes, well spotted.