Allow trailing self in more contexts

[mu001999]

As a follow-up PR to #152996, after this PR:

1. Trailing self can appear in paths
2. E0429 will be no longer emitted, use ...::self [as target]; will be equivalent to use ...::{self [as target]};

r? petrochenkov

[mu001999]

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[petrochenkov]

@mu001999 could you write a change description for lang team for this PR, like in the previous PRs?
I'll nominate it once it's written and the other comments are addressed.
@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[mu001999]

Description for lang team

This PR implements the last part of the lang team guidance from #146972 (comment).
It relaxes the rules to allow trailing self in more context.

Concretely, after this PR

1. use foo::self [as bar]; would be accepted and is resolved the same way as use foo::{self [as bar]};, instead of being rejected with E0429.
2. In non-import paths, trailing self would be accepted as well. This will make things like type E = path::to::Enum::self; and pub(in path::to::Mod::self) accepted.

And technically, this PR will not break existing code.

[mu001999]

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