loadConfig({config, options, logger}) {
    return loadBundlerConfig(config, options, logger);
  }
The Bundler works by creating an IdealGraph, which contains a BundleGraph that models bundles
connected to other bundles by what references them, and thus models BundleGroups.

First, we enter `bundle({bundleGraph, config})`. Here, "bundleGraph" is actually just the
assetGraph turned into a type `MutableBundleGraph`, which will then be mutated in decorate,
and turned into what we expect the bundleGraph to be as per the old (default) bundler structure
 & what the rest of Parcel expects a BundleGraph to be.

`bundle({bundleGraph, config})` First gets a Mapping of target to entries, In most cases there is
 only one target, and one or more entries. (Targets are pertinent in monorepos or projects where you
 will have two or more distDirs, or output folders.) Then calls create IdealGraph and Decorate per target.