Warn when SwiftPM is disabled (#188695)

Fixes #183796

Warns users when Swift Package Manager is disabled through either:

- `flutter config --no-enable-swift-package-manager`
- `enable-swift-package-manager: false` in `pubspec.yaml`

The warning is emitted once per Flutter invocation to avoid duplicate
messages.

### Sample terminal output

With Swift Package Manager disabled, Flutter commands print:

```console
$ flutter pub get
Resolving dependencies...
Downloading packages...
Got dependencies!
Disabling Swift Package Manager will not be allowed in a future version of Flutter.
Process finished with exit code 0
```
<img width="834" height="305" alt="Screenshot 2026-07-01 at 1 07 14 PM"
src="https://github.com/user-attachments/assets/52859424-3d9e-431c-bf26-973114aa1874"
/>


Tests:
- `features_config_test.dart`
- `config_test.dart`
- Static analysis