This PR enables the `flutter screenshot` to work outside a Flutter project. 

This works by enabling `ScreenshotCommand` to find target devices not supported by the project. 

Before: 
```bash
$ cd $HOME # not a Flutter directory

$ flutter screenshot

No devices found yet. Checking for wireless devices...

No supported devices connected.

The following devices were found, but are not supported by this project:
sdk gphone64 arm64 (mobile) • emulator-5554 • android-arm64  • Android 13 (API 33) (emulator)
macOS (desktop)             • macos         • darwin-arm64   • macOS 13.3.1 22E772610a darwin-arm64
Chrome (web)                • chrome        • web-javascript • Google Chrome 119.0.6045.105
If you would like your app to run on android or macos or web, consider running `flutter create .` to generate projects for these platforms.
Must have a connected device for screenshot type device
```

After: 

```bash
$ cd $HOME # not a Flutter directory

$ flutter_source screenshot

Screenshot written to flutter_01.png (313kB).
```

Fixes #115790