Added support for externals through peer dependencies

[subhero24]

Allows parcel to not bundle the peerDependencies of a project and leave them as external requires. This makes it convenient to use parcel for bundling libraries or packages linked locally.

#144
#3305

It works by skipping the package in the node resolve algorithm if it was specified as a peer dependency (as if it was not there in the first place). The resolve algorithm can thus find the package higher up the tree where it was not specified as a peer dependency. In case it did not resolve (as all instances of the package in the tree are specified as a peer dependency), it will be ignored as a dependency and leave the require statement in place.

I find this useful for consuming local packages that have for example a "react" peer dependency. React should not be bundled multiple times as this makes React complain. Also useful to leave "@babel/runtime" requires in place for packages/libraries that want to depend on the babel runtime.

[DeMoorJasper]

Looks like an interesting idea to use peerDeps for this.

Not entirely sure if this'll get merged as we have a feature lock on Parcel v1 to put all our focus on Parcel v2 and have the same amount or more features in Parcel 2 without Parcel 1 and 2 getting too much out of sync

[devongovett]

Would you like to take a shot at implementing this in v2? The resolver is here: https://github.com/parcel-bundler/parcel/blob/v2/packages/resolvers/default/src/DefaultResolver.js#L143. You can return isExcluded: true to prevent a module from being bundled, so it should be even simpler than in v1. I don't think we need a CLI flag for this.

Going to close this PR. We aren't adding any new features to v1 at this point.

[subhero24]

Ok, I will give this a go for v2