Add content/product tag matching for content plugins.

model.find_content compares the required tags on
products with provided product tags, ala repolib.
I'm not a huge fan of this, but it seems to work. I'm not sure there is a great place to do apply Product
information driven logic. In some ways, it seems like it should be handled by the entitlement source and hidden from the plugin, but then, I could see why it might be useful for a future plugin.  An EntSource with a prod_tags attr is enough for current plugins and repolib though, and it's simple.