[Flutter tools] use dwds API to map package: uri paths to server paths in WebAssetServer

[annagrin]

parent: #1692

Proposed fix

make server paths follow directory structure:packages/<package_directory>/<path from package root to the file on disk>

Fix plan

• flutter tools:
  • use PackageUriMapper dwds API to resolve package: uris in WebAssetServer
  • pass the debugger-module-uris to the frontend server

[bkonyi]

@natebiggs does this issue ring any bells? I'm wondering if it's still relevant or not.