[ty] add legacy namespace package support (#20897)

Detect legacy namespace packages and treat them like namespace packages
when looking them up as the *parent* of the module we're interested in.
In all other cases treat them like a regular package.

(This PR is coauthored by @MichaReiser in a shared coding session)

Fixes https://github.com/astral-sh/ty/issues/838

---------

Co-authored-by: Micha Reiser <micha@reiser.io>

crates/ty_python_semantic/resources/mdtest/import/legacy_namespace.md

@@ -0,0 +1,221 @@
+# Legacy namespace packages
+
+## `__import__("pkgutil").extend_path`
+
+```toml
+[environment]
+extra-paths = ["/airflow-core/src", "/providers/amazon/src/"]
+```
+
+`/airflow-core/src/airflow/__init__.py`:
+
+```py
+__path__ = __import__("pkgutil").extend_path(__path__, __name__)
+__version__ = "3.2.0"
+```
+
+`/providers/amazon/src/airflow/__init__.py`:
+
+```py
+__path__ = __import__("pkgutil").extend_path(__path__, __name__)
+```
+
+`/providers/amazon/src/airflow/providers/__init__.py`:
+
+```py
+__path__ = __import__("pkgutil").extend_path(__path__, __name__)
+```
+
+`/providers/amazon/src/airflow/providers/amazon/__init__.py`:
+
+```py
+__version__ = "9.15.0"
+```
+
+`test.py`:
+
+```py
+from airflow import __version__ as airflow_version
+from airflow.providers.amazon import __version__ as amazon_provider_version
+
+reveal_type(airflow_version)  # revealed: Literal["3.2.0"]
+reveal_type(amazon_provider_version)  # revealed: Literal["9.15.0"]
+```
+
+## `pkgutil.extend_path`
+
+```toml
+[environment]
+extra-paths = ["/airflow-core/src", "/providers/amazon/src/"]
+```
+
+`/airflow-core/src/airflow/__init__.py`:
+
+```py
+import pkgutil
+
+__path__ = pkgutil.extend_path(__path__, __name__)
+__version__ = "3.2.0"
+```
+
+`/providers/amazon/src/airflow/__init__.py`:
+
+```py
+import pkgutil
+
+__path__ = pkgutil.extend_path(__path__, __name__)
+```
+
+`/providers/amazon/src/airflow/providers/__init__.py`:
+
+```py
+import pkgutil
+
+__path__ = pkgutil.extend_path(__path__, __name__)
+```
+
+`/providers/amazon/src/airflow/providers/amazon/__init__.py`:
+
+```py
+__version__ = "9.15.0"
+```
+
+`test.py`:
+
+```py
+from airflow import __version__ as airflow_version
+from airflow.providers.amazon import __version__ as amazon_provider_version
+
+reveal_type(airflow_version)  # revealed: Literal["3.2.0"]
+reveal_type(amazon_provider_version)  # revealed: Literal["9.15.0"]
+```
+
+## `extend_path` with keyword arguments
+
+```toml
+[environment]
+extra-paths = ["/airflow-core/src", "/providers/amazon/src/"]
+```
+
+`/airflow-core/src/airflow/__init__.py`:
+
+```py
+import pkgutil
+
+__path__ = pkgutil.extend_path(name=__name__, path=__path__)
+__version__ = "3.2.0"
+```
+
+`/providers/amazon/src/airflow/__init__.py`:
+
+```py
+import pkgutil
+
+__path__ = pkgutil.extend_path(name=__name__, path=__path__)
+```
+
+`/providers/amazon/src/airflow/providers/__init__.py`:
+
+```py
+import pkgutil
+
+__path__ = pkgutil.extend_path(name=__name__, path=__path__)
+```
+
+`/providers/amazon/src/airflow/providers/amazon/__init__.py`:
+
+```py
+__version__ = "9.15.0"
+```
+
+`test.py`:
+
+```py
+from airflow import __version__ as airflow_version
+from airflow.providers.amazon import __version__ as amazon_provider_version
+
+reveal_type(airflow_version)  # revealed: Literal["3.2.0"]
+reveal_type(amazon_provider_version)  # revealed: Literal["9.15.0"]
+```
+
+## incorrect `__import__` arguments
+
+```toml
+[environment]
+extra-paths = ["/airflow-core/src", "/providers/amazon/src/"]
+```
+
+`/airflow-core/src/airflow/__init__.py`:
+
+```py
+__path__ = __import__("not_pkgutil").extend_path(__path__, __name__)
+__version__ = "3.2.0"
+```
+
+`/providers/amazon/src/airflow/__init__.py`:
+
+```py
+__path__ = __import__("not_pkgutil").extend_path(__path__, __name__)
+```
+
+`/providers/amazon/src/airflow/providers/__init__.py`:
+
+```py
+__path__ = __import__("not_pkgutil").extend_path(__path__, __name__)
+```
+
+`/providers/amazon/src/airflow/providers/amazon/__init__.py`:
+
+```py
+__version__ = "9.15.0"
+```
+
+`test.py`:
+
+```py
+from airflow.providers.amazon import __version__ as amazon_provider_version  # error: [unresolved-import]
+from airflow import __version__ as airflow_version
+
+reveal_type(airflow_version)  # revealed: Literal["3.2.0"]
+```
+
+## incorrect `extend_path` arguments
+
+```toml
+[environment]
+extra-paths = ["/airflow-core/src", "/providers/amazon/src/"]
+```
+
+`/airflow-core/src/airflow/__init__.py`:
+
+```py
+__path__ = __import__("pkgutil").extend_path(__path__, "other_module")
+__version__ = "3.2.0"
+```
+
+`/providers/amazon/src/airflow/__init__.py`:
+
+```py
+__path__ = __import__("pkgutil").extend_path(__path__, "other_module")
+```
+
+`/providers/amazon/src/airflow/providers/__init__.py`:
+
+```py
+__path__ = __import__("pkgutil").extend_path(__path__, "other_module")
+```
+
+`/providers/amazon/src/airflow/providers/amazon/__init__.py`:
+
+```py
+__version__ = "9.15.0"
+```
+
+`test.py`:
+
+```py
+from airflow.providers.amazon import __version__ as amazon_provider_version  # error: [unresolved-import]
+from airflow import __version__ as airflow_version
+
+reveal_type(airflow_version)  # revealed: Literal["3.2.0"]
+```