Limit by entry type in Matrix field
I suggest to refactor again and use dropdown fields for the path to your partials instead of the channel entries with asset fields you currently have. This also keeps your twig files more secure (your asset folder is above the web root!).

Set up three dropdown fields ('pathSideBar', 'pathPrimary' and 'pathFullPage') to do
the grouping and one Matrix Block for each of them. 

{% if block.type == 'widgetSideBar' %}

    {% set widgetPath = '_includes/sidebar/' ~ block.pathSideBar ~ '.html' %}
    {% include widgetPath only %}

{% elseif block.type == 'widgetPrimary' %}

    {# ... #}

{% endif %}