legacy_directory_ownership -> hard error

[Centril]

Closes#37872.

r? @petrochenkov

[bors]

☔ The latest upstream changes (presumably #63234) made this pull request unmergeable. Please resolve the merge conflicts.

[ehuss]

Apologies for butting in, but I have some questions about this.
Is this code path even possible? I can't find any UI tests, and I can't figure out a way to trigger it. It looks to have been mostly removed in #46531. The original tests were further removed via #47298.

[Centril]

I have no idea; I was also surprised there were no tests... I guess crater will show if anyone uses it or not =)

[ehuss]

AFAICT, there are two places where UnownedViaMod can be created: submod_path and InvocationCollector. Both only seem to create it when there is a valid #[path] attribute with a value that fails to produce a file_name() or to_str(), both which shouldn't be possible AFAIK (that is, invalid unicode, or an empty string or root path, all which should fail in other ways).

[Centril]

Rolled up into #63247.