html_insert_resource(+ResourceOrList)// is det[private]

Actually include HTML head resources. Called through html_post//2 from html_requires//1 after rewrite by html_head_expansion/2. We are guaranteed we will only get one call that is passed a flat list of requested requirements. We have three jobs:

1. Figure out all indirect requirements
2. See whether we can use any `aggregate' resources
3. Put required resources before their requiree.