Rename package to spacemacs-themes?

I noticed that `M-x customize-themes` (and other commands that complete over `custom-available-themes`) includes `spacemacs` itself as an option, even though this package does not define a theme with that name. Actually selecting it will produce an error: ``` Undefined Custom theme spacemacs ``` This happens because any files named `*-theme.el` present in `custom-theme-load-path` are considered to contain themes named after the file. Other packages that contain multiple themes get around this problem by having their "root definitions" file be named `-themes` instead of `-theme`, so maybe this package should do the same.