Avoid searching for files in nonexistant dirs

Search paths are determined by appending the specified dir to each
directory in the load_paths. Though each dir in load_paths should exist,
when a dir is specified, most load_paths dirs probably don't include it.

For each dir we skip in this way (requiring one stat through
File.directory?) we save 16 stat calls (2 possible prefixes, and 8
possible extensions) looking for files.

Author: jhawthorn

lib/sassc/rails/importer.rb

@@ -100,6 +100,9 @@ def imports(path, parent_path)
           search_paths.map! do |path|
             File.join(path, specified_dir)
           end
+          search_paths.select! do |path|
+            File.directory?(path)
+          end
         end
 
         search_paths.each do |search_path|