Handling of optional dependencies changed, on nightly-2022-01-20 or later

[taiki-e]

UPDATE: fixed in 0.5.11.

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After namespaced and weak dependency features were stabilized (rust-lang/cargo#10269), the value returned by the cargo metadata seems to have changed, and our tests are broken.

https://github.com/taiki-e/cargo-hack/runs/4876584559?check_suite_focus=true