packages: improve theme detection in installation

pkg/omf/functions/packages/omf.packages.install.fish

@@ -39,7 +39,7 @@ function omf.packages.install -a name_or_url
 
   # If we don't know the package type yet, check if the package is a theme.
   if not set -q package_type
-    test -f $install_dir/fish_prompt.fish
+    test -f $install_dir/fish_prompt.fish -o -f $install_dir/functions/fish_prompt.fish
       and set package_type theme
       or set package_type plugin
   end