Note potential but private items in show_candidates

[xizheyin]

Closing #138626 .
We should add potential private items to give ample hints.
And for the other seemingly false positive pub use crate::one::Foo; should be kept because we don't know if the user wants to import other module's items or not, and therefore should be given the full option to do so.
r? compiler

[rustbot]

Failed to set assignee to compilers: invalid assignee

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[compiler-errors]

@rustbot author

[xizheyin]

@rustbot label -S-waiting-on-author +S-waiting-on-review