def source_partial_exists?
      if @site.respond_to?(:in_source_dir)
        File.exists? @site.in_source_dir("_app.yaml")
      else
        File.exists? Jekyll.sanitized_path(@site.source, "_app.yaml")
      end
    end
def source_partial_exists?
      if @site.respond_to?(:in_source_dir)
        File.exists? @site.in_source_dir("_app.yaml")
      else
        File.exists? Jekyll.sanitized_path(@site.source, "_app.yaml")
      end
    end
Checks if a optional _app.yaml partial already exists