Make transform stateless

[Rich-Harris]

This follows up on @tivac's suggestion in rollup/rollup#658 – rather than tracking which CommonJS features are used and injecting them haphazardly using the intro hook, modules import commonjsHelpers. This is cleaner, and will allow the results of the transformation to be cached.

There is a wrinkle however: the synthetic ID used for the helpers goes through the same resolution process as any other ID. Since in this case that frequently means rollup-plugin-node-resolve, we end up passing trying to resolve whatever ID we use with the Node resolution algorithm, which will obviously fail.

So I'm proposing we adopt the following convention: plugins that generate synthetic helper modules, like this one, should use the null character (\0) in the ID. Plugins that implement resolveId should ignore IDs with the null character. (The reason I'm suggesting NUL is that it's invalid in filenames on both *nix and Windows.)

I've already taken the liberty of updating createFilter and node-resolve.

Any thoughts @rollup/collaborators?

Apologies for the bad git discipline, I managed to roll some extra stuff in here 😕

[eventualbuddha]

I realize this is no worse than it was, but this should be generated so as not to conflict with anything in the file. Could do a simple indexOf with an incrementing suffix rather than trying to be clever with scopes.

[Rich-Harris]

good point – fixed

[Victorystick]

Neat! Prepending a module name with NUL is a decent hack. I'll update TypeScript accordingly.