pdl: Use ‘Scope’ to simplify code generation

This open up the lint::Scope struct to be used across the crate. We
might want to move it to a more general place later.

Test: atest pdl_tests
(cherry picked from https://android-review.googlesource.com/q/commit:6cbec1dbbd79f620ba2948bc75b372b0f6cb0ba7)
Merged-In: If8d10f958aff233dfd89d0ba181ebfcc956db25a
Change-Id: If8d10f958aff233dfd89d0ba181ebfcc956db25a