Enable `flutter screenshot` outside Flutter project directory (#138160)

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
2 files changed
tree: 5cf2b3084245006503e283386e3b2ca7a33522e7
  1. .github/
  2. .vscode/
  3. bin/
  4. dev/
  5. examples/
  6. packages/
  7. .ci.yaml
  8. .gitattributes
  9. .gitignore
  10. analysis_options.yaml
  11. AUTHORS
  12. CODE_OF_CONDUCT.md
  13. CODEOWNERS
  14. CONTRIBUTING.md
  15. dartdoc_options.yaml
  16. flutter_console.bat
  17. LICENSE
  18. PATENT_GRANT
  19. README.md
  20. TESTOWNERS
README.md

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