Add custom dependency resolvers

[webpro]

Add something so (dev) dependencies that are not imported using import or require are also marked as used.

For example, ESLint plugins or Babel presets are listed in configuration files using string literals. Would be nice to detect those and mark them as a used/referenced dependency (and thus also list unused ones).

But it should be trivial to add such custom resolvers. And probably ignore patterns to fill the holes.

[webpro]

v1 is out, so this is in (as plugins)