Fix possible double require of classes

When loading environment berfor tire:import:all, it is possible that
models are already loaded when tire tries to load them. This can cause
redefinition of constants and all other kind of havoc.

This fix first tries to constantize the models to see if they are
already loaded before trying to require them.

See issue karmi#744 for discussion.

lib/tire/tasks.rb

@@ -121,10 +121,12 @@ def import_model(index, klass, params)
 
       puts "[IMPORT] Loading models from: #{dir}"
       Dir.glob(File.join("#{dir}/**/*.rb")).each do |path|
-        require path
-
         model_filename = path[/#{Regexp.escape(dir.to_s)}\/([^\.]+).rb/, 1]
-        klass          = model_filename.camelize.constantize
+        begin
+          klass = model_filename.camelize.constantize
+        rescue NameError
+          require(path) ? retry : raise
+        end
 
         # Skip if the class doesn't have Tire integration
         next unless klass.respond_to?(:tire)