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
* This is completely broken on the Impeller renderer, see: https://github.com/flutter/flutter/issues/135052
* Even on the Skia renderer, this gives a software rasterized screenshot which will absolutely look different from a native rendering screenshot.
I plan to remove this functionality from the engine.
