Verify classic templates work as a fallback to block ones

[priethor]

Description

For the WordPress 5.9 release, we need to ensure themes can work in a hybrid mode so that they load block themes when available but can also use PHP ones as a fallback. For example, a block theme with an archive PHP template but no HTML one should be able to find and use the PHP template.

Step-by-step reproduction instructions

1. Install a block theme.
2. Create two versions of the archive template, both in classic PHP and as a blocks template.
3. Verify the block template is being used.
4. Delete the block template, leaving only the PHP one.
5. Verify the classic template is used.

Screenshots, screen recording, code snippet

No response

Environment info

No response

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes

[skorasaurus]

there is an issue for this already at #24834

[skorasaurus]

on second thought, I am reopening because this is on the 5.9 must haves board and don't have edit access to that; but should consolidate discussion of this into #24834 ; close this issue and then update the 5.9 board with that issue

[priethor]

Closing as duplicate of #24834