[flutter_plugin_tools] Add a federated PR safety check (#4329)

Creates a new command to validate that PRs don't change platform interface packages and implementations at the same time, to try to prevent ecosystem-breaking changes. See https://github.com/flutter/flutter/issues/89518 for context.

Per the explanation in the issue, this has carve-outs for:
- Changes to platform interfaces that aren't published (allowing for past uses cases such as making a substantive change to an implementation, and making minor adjustments to comments in the PI package based on those changes).
- Things that look like bulk changes (e.g., a mass change to account for a new lint rule)

Fixes https://github.com/flutter/flutter/issues/89518

script/tool/lib/src/pubspec_check_command.dart

@@ -210,17 +210,11 @@ class PubspecCheckCommand extends PackageLoopingCommand {
   // Returns true if [packageName] appears to be an implementation package
   // according to repository conventions.
   bool _isImplementationPackage(RepositoryPackage package) {
-    // An implementation package should be in a group folder...
-    final Directory parentDir = package.directory.parent;
-    if (parentDir.path == packagesDir.path) {
+    if (!package.isFederated) {
       return false;
     }
     final String packageName = package.directory.basename;
-    final String parentName = parentDir.basename;
-    // ... whose name is a prefix of the package name.
-    if (!packageName.startsWith(parentName)) {
-      return false;
-    }
+    final String parentName = package.directory.parent.basename;
     // A few known package names are not implementation packages; assume
     // anything else is. (This is done instead of listing known implementation
     // suffixes to allow for non-standard suffixes; e.g., to put several