pdl: Let generate_packet_decl take &[Field]

This moves the conversion to Field up two levels and gives us a higher
level API inside generate_packet_decl.

Test: atest pdl_tests
(cherry picked from https://android-review.googlesource.com/q/commit:96cad742b89a481b565407589f1d5e12f91e2310)
Merged-In: I316fe5b676e051008b985e1f234e8682181da086
Change-Id: I316fe5b676e051008b985e1f234e8682181da086