Gatherer/artifact cleanup

In a plugin world, the gatherers become part of our public API. We know about a few things we’re interested in changing. Let’s capture them here.

• add a new MetaElements gatherer which gets name/content of all of them. This would nullify the following: ThemeColor, Viewport, MetaRobots, MetaDescription, (#7025)
• add a new LinkElements gatherer (…actually introduced in #6694 as ExternalResourceLinks). This could nullify Canonical and Hreflang (#7080)
• add a new AnchorElements gatherer which gets rel/href/target/text of all a elements, nullifies AnchorsWithNoRelNoopener and CrawlableLinks (#7101)
• convert ImageUsage into ImageElements gatherer (#7030)
• convert Scripts into ScriptElements gatherer (#7920)
• …

@patrickhulce you had a few in mind, too, right?

[patrickhulce]

I think the primary goal is getting our artifacts in a state ready and useful for plugins. If someone wants to do some brainstorming what audits might need to know about a page and how we should standardize it, that’d be super useful 😃

[brendankenny]

This is more or less done for 5.0 in PublicGathererArtifacts (and BaseArtifacts). Growing the public artifacts will be an ongoing project.

See the number of links above for PRs where this was done and plugins.md#available-artifacts for developer information on using the artifacts.