CustomWebPart.cs

[ToolboxItemAttribute(false)]
    public class CustomWebPart : Microsoft.SharePoint.WebPartPages.WebPart
    {
        // Visual Studio might automatically update this path when you change the Visual Web Part project item.
        private const string _ascxPath = @"~/_CONTROLTEMPLATES/15/test/CustomControl.ascx";

        private const string RelativeFolderCss = "../ReferencedFiles/css";
        private const string RelativeFolderJs = "..ReferencedFiles/js";
        protected override void CreateChildControls()
        {
            IModuleBinder moduleBinder = new ModuleBinder();

            List<List<HtmlGenericControl>> consolidatedControls = new List<List<HtmlGenericControl>>();

            List<HtmlGenericControl> metaTags = new List<HtmlGenericControl>();
            metaTags = moduleBinder.ControlDefinitionMetaTags();

            consolidatedControls.Add(metaTags);

            List<HtmlGenericControl> javascriptControls = new List<HtmlGenericControl>();
            javascriptControls = moduleBinder.ControlDefinitionJavascripts(RelativeFolderJs);

            consolidatedControls.Add(javascriptControls);

            List<HtmlGenericControl> cssControls = new List<HtmlGenericControl>();
            cssControls = moduleBinder.ControlDefinitionCss(RelativeFolderCss);

            consolidatedControls.Add(cssControls);


            foreach (List<HtmlGenericControl> genericControls in consolidatedControls)
            {
                foreach (HtmlGenericControl htmlControls in genericControls)
                {
                    this.Controls.Add(htmlControls);
                }
            }

            Control control = Page.LoadControl(_ascxPath);
            Controls.Add(control);
        }
    }