Move required import parsing out of lint rule (#12536)

## Summary

Instead, make it part of the serialization and deserialization itself.
This makes it _much_ easier to reuse when solving
https://github.com/astral-sh/ruff/issues/12458.

crates/ruff_python_ast/src/lib.rs

@@ -12,7 +12,6 @@ mod expression;
 pub mod hashable;
 pub mod helpers;
 pub mod identifier;
-pub mod imports;
 mod int;
 pub mod name;
 mod node;