Parcel 2: Runtime plugins and bundle loader support

[devongovett]

This implements support for runtime plugins in core, and the default runtime plugin for browser/node environments as discussed in #2507. It gets a bunch of the async import integration tests passing.

Runtime plugins are passed a Bundle, and can modify it in some way to add assets as needed. For example, the default runtime for JS adds loader runtimes to load bundles on demand for async imports. It will also inject the HMR runtime in the future. Runtime plugins are called in the BundlerRunner just after bundles are created and named, and before packaging.

This deviates from the Parcel 2 RFC in that Loader plugins are replaced by Runtime plugins, which are a bit more flexible and easier to implement in a generic way.

Note that the interface exposed to runtime plugins to add assets is a hack at the moment and we need to decide where it will go. I will be opening up a separate issue to discuss this and the rest of the AssetGraph/BundleGraph interface that we want to expose to plugins.

Summary of changes

• Separates asset graph building from Parcel into its own AssetGraphBuilder class, which can be called to generate an asset graph from anywhere without running the rest of parcel. When watching is enabled, AssetGraphBuilder emits an event to let Parcel know that something was invalidated, and Parcel rebuilds.
• Support creating an asset graph from code rather than from a file. This allows runtime plugins to generate code at build time that isn't stored in a file, and have it go through the same pipeline as the rest of the code. It works by adding an optional code property to a TransformerRequest so that TransformerRunner uses that code rather than reading from a file.
• Fixes node global insertion (e.g. Buffer, process, etc.) so that it will not occur if a local variable of the same name is defined. This was happening with importing lodash for example. Probably should also apply this fix to Parcel 1.
• Fixes an issue where one of two separate bundles which point to the same third bundle did not get a reference in the BundleGraph properly since nodes are not revisited during traversal. Fix is to ensure that all dependencies which have an associated bundle_group are replaced in every bundle.

[padmaia]

Found some things unrelated to the runtime plugins, but we've discussed those. Otherwise LGTM.