check for pubspec instead of lib/ (#107968)

packages/flutter_tools/lib/src/flutter_cache.dart

@@ -98,7 +98,7 @@ class PubDependencies extends ArtifactSet {
       return false;
     }
     for (final Package package in packageConfig.packages) {
-      if (!fileSystem.directory(package.packageUriRoot).existsSync()) {
+      if (!fileSystem.directory(package.root).childFile('pubspec.yaml').existsSync()) {
         return false;
       }
     }

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

@@ -936,7 +936,7 @@ void main() {
 
     expect(await pubDependencies.isUpToDate(fileSystem), false); // dependencies are missing.
 
-    fileSystem.file('.pub-cache/hosted/pub.dartlang.org/example-7.0.0/lib/foo.dart')
+    fileSystem.file('.pub-cache/hosted/pub.dartlang.org/example-7.0.0/pubspec.yaml')
       .createSync(recursive: true);
 
     expect(await pubDependencies.isUpToDate(fileSystem), true);