[ty] Migrate the namespace package module resolver tests to mdtests (#18133)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

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

@@ -0,0 +1,107 @@
+# Namespace package
+
+## Basic namespace package
+
+```toml
+[environment]
+python = "/.venv"
+```
+
+`parent/child/one.py`:
+
+```py
+one = 1
+```
+
+`/.venv/<path-to-site-packages>/parent/child/two.py`:
+
+```py
+two = 2
+```
+
+`main.py`:
+
+```py
+import parent.child.one
+import parent.child.two
+```
+
+`from.py`
+
+```py
+# TODO: This should not be an error
+from parent.child import one, two  # error: [unresolved-import]
+```
+
+## Regular package in namespace package
+
+```toml
+[environment]
+python = "/.venv"
+```
+
+An adapted test case from the
+[PEP420 examples](https://peps.python.org/pep-0420/#nested-namespace-packages). The
+`src/parent/child` package is a regular package. Therefore, `site_packages/parent/child/two.py`
+should not be resolved.
+
+```ignore
+src
+  parent
+    child
+      __init__.py
+      one.py
+.venv/site-packages
+  parent
+    child
+      two.py
+```
+
+`parent/child/__init__.py`:
+
+```py
+```
+
+`parent/child/one.py`:
+
+```py
+one = 1
+```
+
+`/.venv/<path-to-site-packages>/parent/child/two.py`:
+
+```py
+two = 2
+```
+
+`main.py`:
+
+```py
+import parent.child.one
+
+import parent.child.two  # error: [unresolved-import]
+```
+
+## Priority between file and identically named namespace package
+
+If there's a namespace package with the same name as a module, the module takes precedence.
+
+`foo.py`:
+
+```py
+x = "module"
+```
+
+`foo/bar.py`:
+
+```py
+x = "namespace"
+```
+
+```py
+from foo import x
+
+reveal_type(x)  # revealed: Unknown | Literal["module"]
+
+import foo.bar  # error: [unresolved-import]
+```