fix: show qualified paths when type names collide in E0308

[rit3sh-x]

A type mismatch between two same-named types from different modules currently renders with no usable information:

mod foo { pub struct S; }
mod bar { pub struct S; }

fn test(_: foo::S) {
    test(bar::S);
}

Before: expected S, found S
After: expected foo::S, found bar::S

When both sides of the mismatch render to the same string, they are re-rendered with display_source_code, which resolves module paths via find_path. The extra work only happens when the strings are equal, so the common path is unchanged. If either side has no renderable path, both keep their plain names, half-qualified message would read worse than the current one.

Nesting falls out of the existing recursion, so Option<foo::S> vs Option<bar::S> is covered too.

[ChayimFriedman2]

The real fix is to only print the ambiguous paths as fully-qualified, but this is complicated to implement and this PR is a reasonable middle-ground.

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[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[rit3sh-x]

@ChayimFriedman2 I shortened the test suite for this implementation. Let me know if you think anything could be improved or changed.

[ChayimFriedman2]

Thanks!

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