The widget preview scaffold requires that previews are defined within
importable libraries. Since Dart does not support absolute paths for
imports or relative imports outside of the project, it's not possible
for the widget previewer to display previews defined outside of `lib/`.
This change updates the preview detector to ignore libraries that don't
have a package name (e.g., libraries under `test/`) and updates
`packageName` properties to not allow for null to make it clear that
package names are required for previews.
Fixes https://github.com/flutter/flutter/issues/178651
Manually updating test dependencies because the pub autoroller seems
busted, https://github.com/flutter/flutter/issues/180503.
This one required manually updating the pubspec files because `flutter
update-packages --force-upgrade` also seemed unable to deal with the
fact that multiple dependencies across packages had to be updated in
sync. There's probably also something broken with the `flutter
update-packages` command. This is something that used to work.
