Adding initial support for DDC's Library Bundler module system to webdev.

[Markzipan]

Adds support for DDC's new module system, which supports hot reload.

This is currently locked behind the canary flag in webdev and requires uses-hot-reload flags to be enabled on downstream builders.

Notable update:

• Adds the requisite options to downstream builders when --canary is passed to webdev.

  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

• Always requests changed assets from build_daemon
• Adds a special handler at $ROOT/reloaded_sources.json that serves reloaded sources

[davidmorgan]

Looks good, thanks! CI didn't run due to path overrides, please check it's green before merging.

[bkonyi]

LGTM!

[davidmorgan]

Still LGTM :) thanks.