[ty] introduce local variables for from imports of submodules in __init__.py(i) (#21173)

This rips out the previous implementation in favour of a new
implementation with 3 rules:

- **froms are locals**: a `from..import` can only define locals, it does
not have global
side-effects. Specifically any submodule attribute `a` that's implicitly
introduced by either
`from .a import b` or `from . import a as b` (in an `__init__.py(i)`) is
a local and not a
global. If you do such an import at the top of a file you won't notice
this. However if you do
such an import in a function, that means it will only be function-scoped
(so you'll need to do
it in every function that wants to access it, making your code less
sensitive to execution
    order).

- **first from first serve**: only the *first* `from..import` in an
`__init__.py(i)` that imports a
particular direct submodule of the current package introduces that
submodule as a local.
Subsequent imports of the submodule will not introduce that local. This
reflects the fact that
in actual python only the first import of a submodule (in the entire
execution of the program)
introduces it as an attribute of the package. By "first" we mean "the
first time in this scope
(or any parent scope)". This pairs well with the fact that we are
specifically introducing a
local (as long as you don't accidentally shadow or overwrite the local).

- **dot re-exports**: `from . import a` in an `__init__.pyi` is
considered a re-export of `a`
(equivalent to `from . import a as a`). This is required to properly
handle many stubs in the
    wild. Currently it must be *exactly* `from . import ...`.
    
This implementation is intentionally limited/conservative (notably,
often requiring a from import to be relative). I'm going to file a ton
of followups for improvements so that their impact can be evaluated
separately.


Fixes astral-sh/ty#133

Author: Gankra

crates/ruff_db/src/files.rs

@@ -475,6 +475,12 @@ impl File {
         self.path(db).as_str().ends_with("__init__.pyi")
     }
 
+    /// Returns `true` if the file is an `__init__.pyi`
+    pub fn is_package(self, db: &dyn Db) -> bool {
+        let path = self.path(db).as_str();
+        path.ends_with("__init__.pyi") || path.ends_with("__init__.py")
+    }
+
     pub fn source_type(self, db: &dyn Db) -> PySourceType {
         match self.path(db) {
             FilePath::System(path) => path