Add issue template and other fixes

Author: mosuem

.github/ISSUE_TEMPLATE/collection.md

@@ -0,0 +1,5 @@
+---
+name: "package:collection"
+about: "Create a bug or file a feature request against package:collection."
+labels: "package:collection"
+---

pkgs/collection/CONTRIBUTING.md

@@ -2,7 +2,7 @@ name: collection
 version: 1.19.0
 description: >-
   Collections and utilities functions and classes related to collections.
-repository: https://github.com/dart-lang/collection
+repository: https://github.com/dart-lang/core/tree/main/pkgs/collection
 
 topics:
  - data-structures