fix: Fix relative path for native namespace packages

Native namespace packages don't have an `__init__.py` module.
This commit adds support for finding relative file path
of such packages.

References: #19, #22.

Author: Shyam Dwaraknath

Diff for: src/pytkdocs/objects.py

@@ -173,20 +173,32 @@ def relative_file_path(self) -> str:
 
         If the relative file path cannot be determined, the value returned is `""` (empty string).
         """
-        top_package_name = self.path.split(".", 1)[0]
-        try:
-            top_package = sys.modules[top_package_name]
-        except KeyError:
+
+        parts = self.path.split(".")
+        namespaces = [".".join(parts[:l]) for l in range(1, len(parts) + 1)]
+        # Iterate through all sub namespaces including the last in case it is a module
+        for namespace in namespaces:
+            try:
+                importlib.import_module(namespace)
+                top_package = sys.modules[namespace]
+            except (ImportError, ModuleNotFoundError, KeyError):
+                # ImportError: Triggered if the namespace is not importable
+                # ModuleNotFoundError: Triggered if the namespace is not a module
+                # KeyError: Triggered if the imported package isn't referenced under the same fully qualified name
+                # Namespace packages are importable, so this should work for them
+                return ""
+
             try:
-                importlib.import_module(top_package_name)
-            except ImportError:
+                top_package_path = Path(inspect.getabsfile(top_package)).parent
+                return str(Path(self.file_path).relative_to(top_package_path.parent))
+            except TypeError:
+                # Triggered if getabsfile() can't be found in the case of a Namespace package
+                pass
+            except ValueError:
+                # Triggered if Path().relative_to can't find an appropriate path
                 return ""
-            top_package = sys.modules[top_package_name]
-        top_package_path = Path(inspect.getabsfile(top_package)).parent
-        try:
-            return str(Path(self.file_path).relative_to(top_package_path.parent))
-        except ValueError:
-            return ""
+
+        return ""
 
     @property
     def name_to_check(self) -> str:

Diff for: tests/fixtures/test_namespace/subspace/__init__.py

@@ -0,0 +1 @@
+"The subspace package docstring."

Diff for: tests/test_loader.py

@@ -1,11 +1,14 @@
 """Tests for [the `loader` module][pytkdocs.loader]."""
 
 import sys
+from pathlib import Path
 
 import pytest
 
 from pytkdocs.loader import Loader, get_object_tree
 
+from . import FIXTURES_DIR
+
 
 def test_import_no_path():
     with pytest.raises(ValueError):
@@ -82,6 +85,16 @@ def test_loading_package():
     assert obj.docstring == "The package docstring."
 
 
+def test_loading_namespace_package():
+    loader = Loader()
+    old_paths = list(sys.path)
+    sys.path.append(str(Path(FIXTURES_DIR).resolve()))
+    obj = loader.get_object_documentation("test_namespace.subspace")
+    assert obj.docstring == "The subspace package docstring."
+    assert obj.relative_file_path == "subspace/__init__.py"
+    sys.path = old_paths
+
+
 def test_loading_module():
     loader = Loader()
     obj = loader.get_object_documentation("tests.fixtures.the_package.the_module")