Global paths cannot start with `crate`

[cramertj]

There was a recent regression in the ability to write code like

use {
    crate::{...},
    ...
};

It used to work under the new edition but now reports "failed to resolve. global paths cannot start with crate". cc @eddyb, @petrochenkov

[cramertj]

Additionally, use { self::... }; and use { super::... }; have never worked, and it'd be great if these could be fixed as well (though obviously that's a separate issue from the specific regression here).

[eddyb]

One more radical solution (than the immediate "defer insertion of CrateRoot until you have the first path segment) is to never insert CrateRoot on any edition, and instead treat ModuleOrUniformRoot::UniformRoot like the result of resolve_crate_root, in resolve_ident_in_module_unadjusted, when uniform_paths is not enabled.