Use dependency inversion to get rid of delayed import in eval_expr

[r41k0u]

In

Python-BPF/pythonbpf/expr/expr_pass.py

Line 482 in 158cc42

from pythonbpf.helper import HelperHandlerRegistry, handle_helper_call

We have a delayed import as a quick fix for handling helpers within eval_expr.

It makes sense to have a single eval_expr handle all expressions, but these two components are somewhat tightly coupled. Having them in the same file will make things ugly.

We must use dependency inversion to get rid of this coupling and the delayed import.