XB should provide no components of its own: move all XB's SDCs into the `xb_test_sdc` module

Updating issue summary.

Note to reviewers: please review this as normal, but once this gets to RTBC, instead of merging it, please set it to Postponed. There's a few MRs in the queue that we'll want to merge in ahead of this one rather than requiring tedious merging/rebasing of them.

Moving back to needs work as rebased with latest 0.x and resolved conflicts but there are now some failing tests.

I'm surprised by how fast @thoward216 got this done, and by how easy the diff was to review! 😮🥳

It's very gratifying to see that the hashes remained unchanged — that's a great piece of validation for ComponentInputsEvolutionTest (added in #3523841: Versioned Component config entities (SDC, JS: prop_field_definitions, block: default_setting, all: slots for fallback) + component instances refer to versions ⇒ less data to store per XB field row), because it proves that one use case that @catch was very worried about (ohai!) truly is simple! 👍 (Insofar this is ever simple, of course.)

Also: thank you for the guidance comments such as https://git.drupalcode.org/project/experience_builder/-/merge_requests/1... — those are super helpful! 🙏

Per @effulgentsia at #11:

Note to reviewers: please review this as normal, but once this gets to RTBC, instead of merging it, please set it to Postponed. There's a few MRs in the queue that we'll want to merge in ahead of this one rather than requiring tedious merging/rebasing of them.

I think everything I had in mind when I wrote #11 landed already, so setting this to RTBC, but please re-postpone it if there are other tedious-to-rebase MRs we're still wanting to get in before this one.

#3503412: Allow Content Author to set site's homepage from navigator is in. That's the last big one.

Merged in upstream, this should still be green 🤞

Feels like a major milestone to say goodbye to these SDCs that we've all interacted with a million times — but they're not gone, they're just boxed up now 🤓