fix(doctor): add an alternative path for the Dart plugin (#163215)

It seems like with some recent plugin/IDE version, the path for the Dart
plugin has changed from `<base path>/Dart` to `<base path>/dart`,
causing `flutter doctor` to incorrectly assume that the plugin is not
installed.

Add `dart` as an alternative to the existing `Dart` package name.

Fixes #163214.

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