Fix relative paths in private import suggestions

[qaijuang]

Resolves #157455.

The private import diagnostic was reusing import.module_path for the "import directly" help. That path is relative to the module where the import was written, so a relative import can be suggested from the wrong place.

For the issue repro, this suggested:

use super::public::hi;

from the crate root. The suggestion should be:

use public::hi;

For relative local imports, this PR rebuilds the help from the resolved module path and only emits it if the item and every module segment are accessible from the failing use site. Additionally, this covers the private-ancestor case from #157455 (comment), where suggesting a direct path would still be invalid. Finally, it maintains an external-alias case as a guardrail to ensure super::s::mem is not treated like a local module path.

[rustbot]

r? @ShoyuVanilla

rustbot has assigned @ShoyuVanilla.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: compiler
• compiler expanded to 73 candidates
• Random selection from 20 candidates