fix: proc-macro example from dep no longer affects feature resolution

[weihanglo]

What does this PR try to resolve?

Previously when checking if a dependency is a proc-macro, the v2 feature resolve resolver v2 looks for proc-macro = true for every target of the dependency. However, it's impossible to depend on a non-lib target as a proc-macro.

This fix switches to only check if proc-macro = true for [lib] target for a dependency.

Fixes https://github.com/rust-lang/cargo/issues/13726.

How should we test and review this PR?

Best reviewed commit-by-commit.

Additional information

I didn't really fix the discrepancy between the behavior of example lib and the documentation, which was mentioned in https://github.com/rust-lang/cargo/issues/13726#issuecomment-2101941480.

Thanks @torhovland for the minimal test case.

[rustbot]

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[epage]

@bors r+

[bors]

:pushpin: Commit 0ead10eebf3a92a9734050da2eb3c3aed07f93e3 has been approved by epage

It is now in the queue for this repository.

[bors]

:hourglass: Testing commit 0ead10eebf3a92a9734050da2eb3c3aed07f93e3 with merge cc34b84eadff412559abcaaca0770e32050c3b35...

[bors]

:sunny: Test successful - checks-actions Approved by: epage Pushing cc34b84eadff412559abcaaca0770e32050c3b35 to master...