Add data checker workflow and some data checkers

[orowith2os]

Supersedes https://github.com/flathub/shared-modules/pull/237 and https://github.com/flathub/shared-modules/pull/238

Allows closing https://github.com/flathub/shared-modules/issues/92

Would like a review from (2-3) others before merging.

[orowith2os]

Oh dear, the commit history is a mess. I'll make sure to squash these when I get some approvals.

[orowith2os]

I've tested with gzdoom and the SDL2 manifests, all appears to be working well.

[ranisalt]

Oh dear, the commit history is a mess. I'll make sure to squash these when I get some approvals.

Don't worry, it can be done by the merger. Great job!

[orowith2os]

@barthalion @hfiguiere @wjt lil reminder, would you mind giving me your thoughts on this? Am I good to merge?

[orowith2os]

Shellcheck says it's okay, I've updated the README, and the workflow appears to be correct, going off of the GitHub docs, I think?

[orowith2os]

I think this is going to need more thought and work, and probably some porting to Python - I want to account for cases where submodules rely on generated dependencies too, and it's also possible that the layout of some submodules isn't what this checker expects.

This could also do with some work in the main external-data-checker too.

[bbhtt]

I would merge this as is now. I don't mind having it in shell and future improvements or python ports can be done later.

Two things -

1. Commits should be squashed
2. The workflow run should be weekly, shared-modules doesn't have super active maintenance and I don't think piling PRs is a good idea.

[bbhtt]

Handled both since the author is not active. Let's merge this.

[bbhtt]

Seems to be working after a minor fix: https://github.com/flathub/shared-modules/commit/8d84ae268a653163f689e41d9ab441a671754510

https://github.com/flathub/shared-modules/actions/runs/11093647389

These were the first two PRs:

https://github.com/flathub/shared-modules/pull/321

https://github.com/flathub/shared-modules/pull/320