Restructure packages

[Mrtenz]

In the current situation snaps-rpc-methods, snaps-controllers, snaps-utils, etc., are heavily coupled together. snaps-controllers exports functionality from snaps-rpc-methods, snaps-types exports from numerous packages, and so on. This results in all of these packages being included in Snap bundles as well.

Some ideas:

• Move Node.js-specific exports to a /node entrypoint (e.g., @metamask/snaps-utils/node). See @metamask/utils for example.
• Extract execution services (currently in snaps-controllers) into a separate package, and export the Node.js execution services from a separate entrypoint.
• Reconsider the snaps-types and snaps-rpc-methods structure, to maybe remove the dependency from snaps-types on snaps-rpc-methods.
• Separate utils that we expect snaps devs to internal-only utils. Maybe we can introduce a snaps-internals package.

We need to design a new architecture for these packages.