pdl: Extract find_parents helper

Tag: #refactor
Bug: 228306436
Test: atest pdl_tests pdl_rust_generator_tests_{le,be}
(cherry picked from https://android-review.googlesource.com/q/commit:f584f4195345de09b78094391cc2d0299332f72d)
Merged-In: I7c93c6efb68e9cc2bb6660493950eae9022bbe42
Change-Id: I7c93c6efb68e9cc2bb6660493950eae9022bbe42