Delete stray .dart_tool/package_config.json files between root and package dirs.

[sigurdm]

In flutter/devtools#8211 the situation was something like this

  /
   packages/
     pubspec.yaml
     .dart_tool/package_config.json
     package_a/
        pubspec.yaml
        .dart_tool/package_config.json

And after migration to workspaces with a pubspec.yaml at the root it looks like this:

  /
    pubspec.yaml
   .dart_tool/package_config.json
   packages/
     .dart_tool/package_config.json # <---- stray file
     package_a/
        pubspec.yaml

The stray file causes imports in package_a to not resolve correctly.
It is hard to discover because it is in a hidden directory, and typically gitignored.

We are already deleting .dart_tool/package_config.json in the folders of the workspace packages

We should also delete any .dart_tool/package_config.json files in folders from the workspace package up to the root.

[sigurdm]

This is not breaking behavior - as these stray .package_config.json files would shadow the package-resolution, making things not work.