Return {0, -1} for missing files in last_modified_and_size/1

[emancu]

Closes #15258.

last_modified_and_size/1 returns {0, 0} for both missing files and existing 0-byte files with epoch-0 mtime. The {0, 0} branch in stale_external? assumed the file was missing, causing perpetual recompilation.

This changes the sentinel for missing files to {0, -1}, which cannot conflict with any real disk entry, and updates the pattern match in stale_external? accordingly.

[josevalim]

Let’s change the size to be -1 instead. This way it cannot conflict with disk entries.

[emancu]

@josevalim Don't you think that will be worse?

I mean, it changes a public interface Mix.Utils.last_modified_and_size/1

I will prepare a separate PR, in case you prefer to add this as a quick fix now, and change the interface in a bigger release.

[josevalim]

Mix.Utils module is private. We can change it as we prefer.

[emancu]

@josevalim Ok, I will work on that fix then, and update the descriptions.

[emancu]

@josevalim Thanks, it looks even simpler :)