fix: --theme-dirs argument to compile_sass management command

This fixes the ability to pass custom theme directories to
the management command which compiles site themes, a la:

   ./manage.py lms compile_sass --theme-dirs /my/custom/themes/dir

The exception, which was due to a incompatible use of @lru_cache, was:

   File "openedx/core/djangoapps/theming/management/commands/compile_sass.py",
   line 93, in parse_arguments:
     available_themes.update({t.theme_dir_name: t for t in get_themes([theme_dir])})
   TypeError: unhashable type: 'list'

This has been broken since the @lru_cache decorator was added, but it
wasn't noticed because:

* We weren't compiling any comprehensive themes in CI.
* Tutor supports compehensive theming, but not *site theming*, so
  it doesn't use this management command at all
  (site themeing == comp theming * site configuration).
* Although edx.org executes this management command, it does not provide
  use the `--theme-dirs` argument, so the bug was not hit.

openedx/core/djangoapps/theming/helpers.py

@@ -268,9 +268,11 @@ def get_themes(themes_dir=None):
     """
     if not is_comprehensive_theming_enabled():
         return []
-    if themes_dir is None:
-        themes_dir = get_theme_base_dirs_unchecked()
-    return get_themes_unchecked(themes_dir, settings.PROJECT_ROOT)
+    if themes_dir:
+        themes_dirs = [themes_dir]
+    else:
+        themes_dirs = get_theme_base_dirs_unchecked()
+    return get_themes_unchecked(themes_dirs, settings.PROJECT_ROOT)
 
 
 def get_theme_base_dirs_unchecked():

openedx/core/djangoapps/theming/management/commands/compile_sass.py

@@ -90,7 +90,7 @@ class Command(BaseCommand):
         if theme_dirs:
             available_themes = {}
             for theme_dir in theme_dirs:
-                available_themes.update({t.theme_dir_name: t for t in get_themes([theme_dir])})
+                available_themes.update({t.theme_dir_name: t for t in get_themes(theme_dir)})
         else:
             theme_dirs = get_theme_base_dirs()
             available_themes = {t.theme_dir_name: t for t in get_themes()}

openedx/core/djangoapps/theming/tests/test_helpers.py

@@ -44,6 +44,7 @@ class TestHelpers(TestCase):
             Theme('red-theme', 'red-theme', get_theme_base_dir('red-theme'), settings.PROJECT_ROOT),
             Theme('stanford-style', 'stanford-style', get_theme_base_dir('stanford-style'), settings.PROJECT_ROOT),
             Theme('test-theme', 'test-theme', get_theme_base_dir('test-theme'), settings.PROJECT_ROOT),
+            Theme('empty-theme', 'empty-theme', get_theme_base_dir('empty-theme'), settings.PROJECT_ROOT),
         ]
         actual_themes = get_themes()
         self.assertCountEqual(expected_themes, actual_themes)