Remove unnecessary imports of 'package:collection' IterableExtension

[oprypin]

• Replace .whereNotNull() with .nonNulls which is now in Dart core.
• .firstOrNull, .lastOrNull, .singleOrNull and .elementAtOrNull(i) are also in Dart core and even under the same name, so simply drop the import of 'package:collection' whenever possible.

Not updating the changelog because there's 0 visible change to users.

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[srawlins]

Thanks!