Debug extension directories should be unnested from `dwds/`

[elliette]

Currently the structure is:

• webdev/**
• dwds/**
  • dart_debug_extension/**
• frontend_server_common
• etc

We should move the debug extension to be its own top-level directory.

Note: Might make sense to do this refactoring after the legacy Dart Debug Extension directory is deleted

[bkonyi]

This should be done before we move packages from this repo into the Dart SDK, assuming we decide to do so.