This reverts commit a27cda2.

Fixes three NameErrors which were introduced by the previous commit,
leading to it being reverted.

These NameErrors slipped through because the script was not tested on
any themes that had zero SCSS overrides.

This will be used to protect against bugs like the on fixed in the
previous commit.

This will help safe-guard against bugs in the Sass and Webpack build
which only arise from certain uses of the legacy Comprehensive Theming
system.

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.