Update schema validation to disallow multiple commands with the same ID

[DragaDoncila]

In #374 I discovered that our auto-conversion of npe1 plugins could generate multiple commands with the same ID, and this didn't run into any errors until trying to launch napari with the plugin installed, when app-model would balk at registering the second command.

Initially I thought "well that's ok users shouldn't run into this because the schema validator should error on their manifest", but it doesn't! In the current implementation, the manifest below is valid:

name: napari-demo
...
contributions:
  commands:
    - id: napari-demo.about
      title: About napari demo plugin
      python_name: napari_demo:about
    - id: napari-demo.about
      title: Other about thing
      python_name: napari_demo:hello_world

We should update our tests and our schema validator to raise an error when encountering a manifest with multiple commands with the same command ID.