Allow all `github:.../*.yaml` references to be symlinks or redirects

[jandubois]

While working on the tests for #4159 I tripped myself a few times over the problem that except for the root redirect of github:jandubois, all others need to specify a trailing slash to mark them as directories:

❯ l tmpl url github:jandubois//redirect
https://raw.githubusercontent.com/jandubois/jandubois/main/redirect.yaml

❯ l tmpl url github:jandubois//redirect/
https://raw.githubusercontent.com/jandubois/lima/master/templates/default.yaml

Without the trailing slash redirect is treated as redirect.yaml and cannot redirect to another repo, even if redirect.yaml doesn't exist.

If I can accidentally forget to add the slash while working on the tests, then it is very likely that casual users will also run into it¹.

I think we can do 2 things to mitigate it:

1. Treat foo as foo/ if foo.yaml does not exist (and foo has no extension).

   So we need to make at least a HEAD request for foo.yaml, and if that fails we decide that this was meant to look up foo/.lima.yaml instead.

2. Allow all *.yaml references (only in ORG repos?) to be symlinks or redirects

   This has the advantage that the user doesn't have to create extra subdirectories as containers for .lima.yaml, as there is likely nothing else in there. Instead foo.yaml could contain the redirect URL (or symlink).

I think (2) is more useful than (1), but if we do (2) and already fetch the file, then we can just do (1) at the same time at no extra cost.

It is just a fallback when foo.yaml doesn't exist, so the alternatives are either return an error right away, or retry the foo/.lima.yaml symlink as well.

I think we should implement both (1) and (2). For consistency we should probably also implement them for non-ORG repos, but of course they would only allow symlinks and not redirects to other repos.

Originally posted by @jandubois in #4159 (comment)

Footnotes

1. For similar reasons many websites redirect http://example.com/ to http://example.com (or vice versa). ↩

[jandubois]

I believe the consensus was to implement both (1) and (2), and not restrict them to ORG repos.

Of course redirects via github: only work in ORG repos, same as for .lima.yaml files. Non-ORG repos can only use symlinks within the same repo.

Putting into 2.0 milestone for now, but may be moved to 2.1 if we run out of time.