[red-knot] Rename constraint to predicate (#16382)

In https://github.com/astral-sh/ruff/pull/16306#discussion_r1966290700,
@carljm pointed out that #16306 introduced a terminology problem, with
too many things called a "constraint". This is a follow-up PR that
renames `Constraint` to `Predicate` to hopefully clear things up a bit.
So now we have that:

- a _predicate_ is a Python expression that might influence type
inference
- a _narrowing constraint_ is a list of predicates that constraint the
type of a binding that is visible at a use
- a _visibility constraint_ is a ternary formula of predicates that
define whether a binding is visible or a statement is reachable

This is a pure renaming, with no behavioral changes.

crates/red_knot_python_semantic/src/semantic_index.rs

@@ -25,10 +25,10 @@ use crate::Db;
 pub mod ast_ids;
 pub mod attribute_assignment;
 mod builder;
-pub(crate) mod constraint;
 pub mod definition;
 pub mod expression;
 mod narrowing_constraints;
+pub(crate) mod predicate;
 pub mod symbol;
 mod use_def;
 mod visibility_constraints;