[red-knot] Support stub packages (#17204)

## Summary

This PR adds support for stub packages, except for partial stub packages
(a stub package is always considered non-partial).

I read the specification at
[typing.python.org/en/latest/spec/distributing.html#stub-only-packages](https://typing.python.org/en/latest/spec/distributing.html#stub-only-packages)
but I found it lacking some details, especially on how to handle
namespace packages or when the regular and stub packages disagree on
whether they're namespace packages. I tried to document my decisions in
the mdtests where the specification isn't clear and compared the
behavior to Pyright.

Mypy seems to only support stub packages in the venv folder. At least,
it never picked up my stub packages otherwise. I decided not to spend
too much time fighting mypyp, which is why I focused the comparison
around Pyright

Closes https://github.com/astral-sh/ruff/issues/16612

## Test plan

Added mdtests

crates/red_knot_python_semantic/src/module_resolver/module.rs

@@ -96,6 +96,9 @@ impl ModuleKind {
     pub const fn is_package(self) -> bool {
         matches!(self, ModuleKind::Package)
     }
+    pub const fn is_module(self) -> bool {
+        matches!(self, ModuleKind::Module)
+    }
 }
 
 /// Enumeration of various core stdlib modules in which important types are located