[SwiftPM] Rename SWIFT_PACKAGE_MANAGER environment variable to FLUTTER_SWIFT_PACKAGE_MANAGER (#159502)

Previously, the Swift Package Manager feature could be overridden using
the `SWIFT_PACKAGE_MANAGER` environment variable.

This environment variable name is a bit generic and might collide with
other tooling.

This renames the environment variable to
`FLUTTER_SWIFT_PACKAGE_MANAGER`.

**This is a breaking change**, however, [we haven't documented this
environment
variable](https://docs.flutter.dev/packages-and-plugins/swift-package-manager/for-app-developers#how-to-turn-on-swift-package-manager).

Addresses flutter/flutter#159121

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Author: loic-sharma

packages/flutter_tools/lib/src/features.dart

@@ -185,7 +185,7 @@ const Feature previewDevice = Feature(
 const Feature swiftPackageManager = Feature(
   name: 'support for Swift Package Manager for iOS and macOS',
   configSetting: 'enable-swift-package-manager',
-  environmentOverride: 'SWIFT_PACKAGE_MANAGER',
+  environmentOverride: 'FLUTTER_SWIFT_PACKAGE_MANAGER',
   master: FeatureChannelSetting(
     available: true,
   ),

packages/flutter_tools/test/general.shard/features_test.dart

@@ -403,13 +403,23 @@ void main() {
       expect(nativeAssets.stable.available, false);
     });
 
-    test('${swiftPackageManager.name} availability and default enabled', () {
-      expect(swiftPackageManager.master.enabledByDefault, false);
-      expect(swiftPackageManager.master.available, true);
-      expect(swiftPackageManager.beta.enabledByDefault, false);
-      expect(swiftPackageManager.beta.available, true);
-      expect(swiftPackageManager.stable.enabledByDefault, false);
-      expect(swiftPackageManager.stable.available, true);
+    group('Swift Package Manager feature', () {
+      test('availability and default enabled', () {
+        expect(swiftPackageManager.master.enabledByDefault, false);
+        expect(swiftPackageManager.master.available, true);
+        expect(swiftPackageManager.beta.enabledByDefault, false);
+        expect(swiftPackageManager.beta.available, true);
+        expect(swiftPackageManager.stable.enabledByDefault, false);
+        expect(swiftPackageManager.stable.available, true);
+      });
+
+      test('can be enabled', () {
+        platform.environment = <String, String>{
+          'FLUTTER_SWIFT_PACKAGE_MANAGER': 'true',
+        };
+
+        expect(featureFlags.isSwiftPackageManagerEnabled, isTrue);
+      });
     });
   });
 }