pdl: Rework reading of scalar fields

This CL reverts the code reading scalar fields to match the logic in
the Python backend. This approach will make it simpler to update the
two backends going forward.

Test: atest pdl_tests pdl_rust_generator_tests_{le,be}
Change-Id: I49ddb3dae8eab1bb00cf730eda50a051d326ce2c