Allow showing output

[datapythonista]

xref PyO3/maturin#1243

Commands using cargo_metadata can't show the cargo output, which makes problems pretty hard to diagnose if something goes wrong, like cargo being blocked.

For example, when building a Python extension project with maturin using maturin build, and a lock file ~/.cargo/.package-cache exists, the command will show no output, and stay idle forever. Being able to show the cargo output Blocking waiting for file lock on the registry index to the user would be extremely helpful.