57704 Use pub inside the Flutter directory (cla: yes, team, waiting for tree to go green)

57821 fix a typo in trace events for the image cache (cla: yes, framework, team, waiting for tree to go green)

57871 [flutter_tools] rename output LICENSE file to NOTICES and support loading either (cla: yes, framework, team, tool)

58018 Prevent building non-android plugins in build aar (cla: yes, team, tool, waiting for tree to go green)

58027 Link to migration guide template from pull request template (cla: yes, team, waiting for tree to go green)

58050 Flutter 1.17.2 cherrypicks (a: accessibility, cla: yes, d: examples, engine, f: cupertino, f: material design, framework, team, tool, work in progress; do not review)