pdl: Fix generation of aliased child packet parsing

The generator did not handle well packets with child aliases,
as it was generating a wildcard with no constraint for
the child specialization.

Extend the generator to handle such cases, within a limit
- fields must have constraints in immediate children
  (i.e. cannot generate matcher for fields if they are constrained
   by granchildren only)
- double aliased packets are not allowed (i.e. packets with no
  constraints for two generations)

Bug: 279465240
Test: atest pdl_generated_files_compile
Change-Id: If4c3a82e8a06edeab4ef54a8c109b994f6a2dc25