syntax: Make ViewItemUse no longer contain multiple view paths.

It reflected the obsolete syntax `use a, b, c;` and did not make past the parser (though it was a non-fatal error so we can continue). This legacy affected many portions of rustc and rustdoc as well, so this PR cleans them up altogether.

As a side effect of cleanup, we now have `SCHEMA_VERSION` in `rustdoc::clean` (instead of the crate root), so it has a better chance to be updated when `rustdoc::clean` gets updated.