pdl: move code generators into ‘backends’ module

The Rust generator is now nearly 1k lines long, so it’s time to move
it into a more deeply nested module structure. As a first step, I’ll
move it to a ‘backends’ module, and then we can expand further from
there.

Test: m pdl
Change-Id: I22550ecaefe9bbab0e9c2c020d23a9f0874a8716