rustc_public: import requried extern crates in run_driver!

[makai410]

It's unfair to put users to do this work, since they don't know what the code would be after macro expansion and what crates should be imported if they don't take a look at the source code.

However, unfortunately, they have to enable rustc_private themselves for now because run_driver! could occur as an expression, which would trigger the stmt_expr_attributes if we enable rustc_private in the macro. I think this would be fixed after we have a stable driver.

[rustbot]

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[rustbot]

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a