[linter] Handle linter migration in generate_package_config.dart

The linter repository is moving into the SDK repo, moving
from third_party/pkg/linter to pkg/linter. Make generate_package_config.dart compatible with a non-breaking migration, that adds the package in the new location, changes Flutter and Dart
to use the new location, then removes the copy from the old location.

Bug: dart-lang/linter#4411
Change-Id: I0653562a8af09a06582bbf17a44766fa64e2881f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/321722
Reviewed-by: Alexander Thomas <athom@google.com>
Commit-Queue: William Hesse <whesse@google.com>

tools/generate_package_config.dart

@@ -82,7 +82,16 @@ void main(List<String> args) {
   var hasDuplicatePackages = false;
 
   for (var name in uniqueNames) {
-    var matches = packages.where((p) => p.name == name).toList();
+    var matches = [
+      for (final p in packages)
+        if (p.name == name) p
+    ];
+    if (name == 'linter' && matches.length > 1) {
+      final oldLinter = matches
+          .firstWhere((p) => p.rootUri.endsWith('third_party/pkg/linter'));
+      packages.remove(oldLinter);
+      matches.remove(oldLinter);
+    }
     if (matches.length > 1) {
       print('Duplicates found for package:$name');
       for (var package in matches) {