Provide a default if pkg.main is missing

[albertogasparin]

I was trying to use babel-plugin-lodash with date-fns, however the plugin breaks badly because date-fns package.json does not have a main attribute.

Looking a the code, it already handles the case where package.json is missing. So, I'd like to propose to extend the check to the main attribute too, defaulting to index.js if missing so the plugin could be more resilient.

[jdalton]

👌 Would you be up for making a PR?

[albertogasparin]

Sure, will do