Glues FES into build_runner, which is our first step towards hot reload + build_runner.
The high level workflow is:

1. A persistently running frontend server is initialized once when a build starts.
2. build_runner requests JS files based on locally modified/generated dart files (as usual). Builders that collect meta-information about DDC modules also - as a side effect - record the main app entrypoint and any locally modified files.
3. When a JS file is requested, the frontend server receives recompile requests via a proxy queue (to maintain communication order with the frontend server).
4. The frontend server processes compilation requests and serves compiled JS files back to build_runner (hot-reload ready).

Major changes:

• Adds a DdcFrontendServerBuilder to our set of DDC builders (enabled via the web-hot-reload config). This builder keeps a PersistentFrontendServer instance alive across rebuilds. Compile/recompile requests are queued via a FrontendServerProxyDriver resource.
• Uses scratch_space to record both 1) the main app entrypoint and 2) updated local files from the entrypoint_marker builder and the module_builder builder respectively. These are side effects that break certain stateful 'guarantees' of standard build_runner execution. The entrypoint_marker builder runs before any of the downstream DDC builders and finds the web entrypoint, as Frontend Server must receive the same entrypoint on every compilation request.
• Requires that strongly connected components in both the frontend server and build_runner be disabled.

Test changes:

• Extends build_test to permit incremental builds. This involves passing the asset graph + asset reader/writer across build results and only performing cleanup operations after a series of rebuilds.
• build_test doesn't support runs_before and other ordering rules in build.yaml, so the above changes allows a kind of imperative ordering, which is important for testing entrypoint_marker.

Minor changes:

• Added a flag to disable strongly connected components in build_web_compilers (implemented using raw ddc meta-modules over clean ddc meta-modules + enforcing fine module aggregation).
• Added disposal logic to scratch_space so that rebuilds only retain modified files.
• Updated scratch_space package_config.json specs (packageUri and rootUri). The previous values didn't seem to make sense to me, but I'm also not familiar with how that's standardized in scratch_space.
• Added file and uuid deps to build_modules.
• Moved around some helper functions.
• Ported some naming functions from the DDC runtime.

Currently doesn't support live-reloading (functionality appears to have been broken a while ago). This'll be added in an upcoming change and permit webdev-like auto-hot-reload on save (on top of manual).

Enable this by adding the following to a project's build.yaml:

global_options:
  build_web_compilers|sdk_js:
    options:
      web-hot-reload: true
  build_web_compilers|entrypoint:
    options:
      web-hot-reload: true
  build_web_compilers|entrypoint_marker:
    options:
      web-hot-reload: true
  build_web_compilers|ddc:
    options:
      web-hot-reload: true
  build_web_compilers|ddc_modules:
    options:
      web-hot-reload: true