Bundler Rewrite: Experimental Bundler

[gorakong]

↪️ Pull Request

This is a draft PR for the bundler rewrite as per the prototype in https://github.com/devongovett/bundler-algorithm/blob/master/src/main.rs . Changes are mostly in ExperimentalBundler.js. The prototype generates what we call an “Ideal Bundle Graph” that is maximally deduplicated. We replicated this "ideal graph", and then backported it to our previous bundle graph structure.

Things to note:

• Within the context of the ideal graph, bundle groups are simply groupings of bundles and not any physical nodes.
• BundleRoots represent a bundle's entry asset (not to be confused with a project's entry). Not all assets are bundleRoots.
• We have many auxiliary data structures that we’re hoping to phase out once we pass all integration tests.

🚨 Test instructions

To run tests using the Experimental Bundler, use the flag PARCEL_TEST_EXPERIMENTAL_BUNDLER=1.