Teach the scanner about search_api indexes and myfacet facets

Problem/Motivation

A listing endpoint is served either by an entity query or by a search index, and the difference is not cosmetic: facets, relevance sorting and full-text filtering exist only in the second case. Which one applies to a bundle is a property of the site, and the scanner is the only thing that can find out.

Proposed resolution

• When search_api is installed, record the indexes, their datasources, their indexed fields and which bundles they cover.
• When myfacet is installed, record the facets defined against those indexes.
• Express the result as a suggestion for the listing source of each bundle, not as a decision — it becomes a facets role the site confirms.
• Without either module the section is simply absent and every listing plans as an entity query.

Remaining tasks

Everything.