python: make setup.py work even if imported from elsewhere.

I feel like I'm stepping into unknown waters here :D

src/python/setup.py.cmake

@@ -65,6 +65,12 @@ setup(
     name='magnum',
     packages=packages,
     ext_modules=[Extension(name, sources=[]) for name, path in extension_paths.items() if path],
+    package_dir={
+        # Explicitly supply package_dir so importing this file from another
+        # setup.py (i.e., when this is a bundled dependency of another project)
+        # does the right thing
+        '.': os.path.dirname(__file__)
+    },
     cmdclass={
         'build_ext': TheExtensionIsAlreadyBuiltWhyThisHasToBeSoDamnComplicated
     },