[cupertino_ui,material_ui] Post-import adjustments for new package-based structure.

A single squashed commit of all work after copying the Material and
Cupertino libraries, and related code, into material_ui and
cupertino_ui. Puts the code in the right place and sets it all up
properly in the new repo so that it passes CI.

 * Copied code was moved to material_ui and cupertino_ui.
 * Imports of flutter/material.dart and flutter/cupertino.dart were
   changed to material_ui/material_ui.dart and
   cupertino_ui/cupertino_ui.dart, respectively.
 * Relative imports were updated to new paths or packages to work in
   their new location.
 * The flutter_goldens package was referenced from
   script/flutter_goldens instead of the flutter/flutter sdk version.
 * Dependencies referenced with `any` version were updated to the
   caret version in flutter/flutter's lock file.
 * Examples were set up as their own packages in the `example` directory.
 * READMEs were updated to be consistent with the new setup.
 * Pubspecs were updated to work with the new setup. Set to Flutter
   3.44.0, which is the stable version first containing the code freeze.
 * Dart data-driven fixes were set up in the fix_data directories along
   with their tests in the test_fixes directories.
 * Doc imports were updated to work in the new setup.
 * One dart fix test was removed since it was already tested in
   flutter/flutter's widgets library.
 * The drag_target dart fix was deleted and kept in flutter/flutter.
 * Other widgets-only dart fixes were deleted in cupertino_ui for the
   same reason.
 * Tests with cross imports were skipped and moved to the
   temporarily_skipped_tests directories, to be fixed before 1.0.0.
 * Formatting was adjusted to pass analysis here.
 * A usage of flutter_test's byTooltip finder was removed since it
   contains a cross import of material.
 * License headers were updated to match this repository.
 * Batch release was disabled for now (until we're ready to release
   1.0.0).
 * Exceptions were added for some pinned dependencies that came from
   flutter/flutter.
 * Some fixes and skips were added to the example tests in order to be
   able to run them on Chrome. It seems that these tests were never run
   on web in flutter/flutter.