Support DevTools extension detection for pub workspaces

[kenzieschmoll]

For DevTools extensions, we currently assume that the .dart_tool/package_config.json file will always live in the project root. This may not be true for pub workspaces AKA monorepos.

What we should instead do is modify the DevTools server extension APIs to take the path to the package config file instead of the project root.

Related:

• Add a way to find the package_config.json file for a given pubspec.yaml file dart-lang/pub#4218