[ty] Rename Type unwrapping methods (#20857)

## Summary

Rename "unwrapping" methods on `Type` from e.g.
`Type::into_class_literal` to `Type::as_class_literal`. I personally
find that name more intuitive, since no transformation of any kind is
happening. We are just unwrapping from certain enum variants. An
alternative would be `try_as_class_literal`, which would follow the
[`strum` naming
scheme](https://docs.rs/strum/latest/strum/derive.EnumTryAs.html), but
is slightly longer.

Also rename `Type::into_callable` to `Type::try_upcast_to_callable`.
Note that I intentionally kept names like
`FunctionType::into_callable_type`, because those return `CallableType`,
not `Option<Type<…>>`.

## Test Plan

Pure refactoring

crates/ty_python_semantic/src/place.rs

@@ -1447,7 +1447,7 @@ mod implicit_globals {
     fn module_type_symbols<'db>(db: &'db dyn Db) -> smallvec::SmallVec<[ast::name::Name; 8]> {
         let Some(module_type) = KnownClass::ModuleType
             .to_class_literal(db)
-            .into_class_literal()
+            .as_class_literal()
         else {
             // The most likely way we get here is if a user specified a `--custom-typeshed-dir`
             // without a `types.pyi` stub in the `stdlib/` directory