Allow the mainFields option

[crubier]

Allow to use things like mainFields: ['module', 'main'] in options

See issue: #2

[arcanis]

Hey @crubier, thanks for working on this! I'd prefer to avoid reimplementing the whole resolution, so that this plugin can focus on the PnP part (ie transforming lodash into /path/to/lodash, but not the file itself). Otherwise we would have to use the same file loading mechanisms as the rest of Rollup, etc.

What do you think of this alternative approach?

• If the importee is an absolute path, bailout
• Otherwise, resolve the importee through resolveToUnqualified and return it by forwarding the unqualified resolution to this.resolve (cf Chain plugins resolveId functions rollup/rollup#1356 (comment))

This way I think that the "main field" resolution will be handled by Rollup's core, which will be less work for us! Does that sound like a plan?

[crubier]

I agree I am not proud of this MR. I definitely reinvented the wheel in the last few commits, but I just really needed to get this to work quickly for my internal tools (and it does work).

Your idea sounds great, but I don't totally see what I should do to put it in application right now. I will give it a try asap though. I understand that I can access rollup's own functions (aka hooks) from the this object in the plugin.