Add ErrorKind::DirNotEmpty

[rouge8]

This patch adds the ability to express ENOTEMPTY as an io::Error.

I followed #84744 to see what needed to be updated.

[rust-highfive]

Some changes occurred in src/tools/clippy.

cc @rust-lang/clippy

[rust-highfive]

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[the8472]

If approved #79965 would make this redundant.

[rouge8]

If approved #79965 would make this redundant.

Ah that looks like a more comprehensive approach!