[ty] Refactor completion generation

This refactor is intended to give more structure to how we generate
completions. There's now a `Context` for "how do we figure out what kind
of completions to offer" and also a `CollectionContext` for "how do we
figure out which completions are appropriate or not." We double down on
`Completions` as a collector and a single point of truth for this. It
now handles adding information to `Completion` (based on the context)
and also skipping completions that are inappropriate (instead of
filtering them after-the-fact).

We also bundle a bunch of state into a new `ContextCursor` type, and
then define a bunch of predicates/accessors on that type that were
previously free functions with loads of parameters.

Finally, we introduce more structure to ranking. Instead of an anonymous
tuple, we define an explicit type with some helper types to hopefully
make the influence on ranking from each constituent piece a bit clearer.

This does seem to fix one bug around detecting the target for non-import
completions, but otherwise should not have any changes in behavior.

This is meant to be a precursor to improving completion ranking.

crates/ty_python_semantic/src/semantic_model.rs

@@ -380,7 +380,7 @@ pub struct MemberDefinition<'db> {
 /// single-underscore names. This matches the order of the variants defined for
 /// this enum, which is in turn picked up by the derived trait implementation
 /// for `Ord`.
-#[derive(Clone, Copy, Eq, PartialEq, PartialOrd, Ord)]
+#[derive(Clone, Copy, Debug, Eq, PartialEq, PartialOrd, Ord)]
 pub enum NameKind {
     Normal,
     Dunder,