Remove pubspec.lock files from packages

The dart team recommends not committing the pubspec.lock in regular
packages. The examples should still have the lock files to "ensures
changes to transitive dependencies are explicit".

https://dart.dev/guides/libraries/private-files#pubspec-lock

.gitignore

@@ -6,3 +6,4 @@
 .vscode
 .DS_Store
 **/*/build
+flutter_filament_federated/*/pubspec.lock