feat: add util.resource_path() function

need that now that we have configurable mako template paths

tests/test_util.py

@@ -277,3 +277,43 @@ class TestProgressLoop(TestCase):
         # without progress
         mod.progress_loop(act, [1, 2, 3], None,
                           message="whatever")
+
+
+class TestResourcePath(TestCase):
+
+    def test_basic(self):
+
+        # package spec is resolved to path
+        path = mod.resource_path('wuttjamaican:util.py')
+        self.assertTrue(path.endswith('wuttjamaican/util.py'))
+
+        # absolute path returned as-is
+        self.assertEqual(mod.resource_path('/tmp/doesnotexist.txt'), '/tmp/doesnotexist.txt')
+
+    def test_basic_pre_python_3_9(self):
+
+        # the goal here is to get coverage for code which would only
+        # run on python 3.8 and older, but we only need that coverage
+        # if we are currently testing python 3.9+
+        if sys.version_info.major == 3 and sys.version_info.minor < 9:
+            pytest.skip("this test is not relevant before python 3.9")
+
+        from importlib.resources import files, as_file
+
+        orig_import = __import__
+
+        def mock_import(name, globals=None, locals=None, fromlist=(), level=0):
+            if name == 'importlib.resources':
+                raise ImportError
+            if name == 'importlib_resources':
+                return MagicMock(files=files, as_file=as_file)
+            return orig_import(name, globals, locals, fromlist, level)
+
+        with patch('builtins.__import__', side_effect=mock_import):
+
+            # package spec is resolved to path
+            path = mod.resource_path('wuttjamaican:util.py')
+            self.assertTrue(path.endswith('wuttjamaican/util.py'))
+
+            # absolute path returned as-is
+            self.assertEqual(mod.resource_path('/tmp/doesnotexist.txt'), '/tmp/doesnotexist.txt')