remove import/ignore + use UnambiguousJSGrammar

[benmosher]

Assume any module with at least one import or export (including export {}) is a module.

Ignore any module failing this.

Keep import/ignore for things like ignoring CoffeeScript, but it will be empty by default. Files matching ignore patterns will be completely ignored, as if they were core modules or CJS.

Technically, this is how ignored files work today; if exports are detected, they are parsed anyway. This reversal makes more sense.

Supersedes: #268
Idea thanks to: #384

[benmosher]

related: #268

[benmosher]

Original issue text:

Assuming a decent, fast, reliable CommonJS regex can be constructed, import/ignore can potentially cease to exist.

Modules would be ignored if they both

• match the CommonJS regex
  and

• are not definitively a module (i.e. match the ES6 module regex or otherwise qualify as a module, see Support *.jsm and/or package.json glob field #237 or Use Rollup's 'jsnext:main' to collect names #103)

  Can't simply not qualify as a module, as this would ignore errors where a file doesn't (yet) export any names and another module attempts to import from it.

Can't think of why else ignore would be needed.

Definitely a breaking change.

[benmosher]

@jfmengels: let me know what you think of the new issue? if I've missed some key case?