Replace WC_TEMPLATE_PATH with WC()->template_path()

This replaces the deprecated constant WC_TEMPLATE_PATH with WC()->template_path().

Because this constant is created when Woocommerce is instantiated, we can not apply filters to overwrite its value in our themes or from plugins that are loaded after woocommerce. 

This change will solve that.

Same thought for TEMPLATEPATH and STYLESHEETPATH