[pkg] ensure each publishable package has a repository pubspec field

Change-Id: I06c0d10c87c3f329098352a7abe9ef9fa0b1532e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/241504
Reviewed-by: Kevin Moore <kevmoo@google.com>
Reviewed-by: Nate Bosch <nbosch@google.com>
Commit-Queue: Devon Carew <devoncarew@google.com>

pkg/dart_internal/pubspec.yaml

@@ -1,10 +1,10 @@
 name: dart_internal
 version: 0.2.5
-repository: https://github.com/dart-lang/sdk/tree/master/pkg/dart_internal
 description: >-
   This package is not intended for wide use. It provides a temporary API to
   solve the problem: "Given an object some generic type A, how do I construct an
   instance of generic type B with the same type argument(s)?"
+repository: https://github.com/dart-lang/sdk/tree/main/pkg/dart_internal
 
 environment:
   # Restrict the upper bound so that we can remove support for this in a later