Auto merge of #10172 - ehuss:doc-lib-before-bin, r=alexcrichton

Auto merge of #10172 - ehuss:doc-lib-before-bin, r=alexcrichton

Document lib before bin.

This changes it so that documenting a library is required to finish before documenting a binary. The issue is that the binary may have intra-doc links to the library. If they are documented concurrently, then the links will sometimes fail (since it is a race).   Or, if doing `cargo doc --bins`, then the library docs wouldn't exist at all.

Note that in the tests this introduces some more name collisions if you just run `cargo doc --bins` and there is a colliding library/binary name. There is some risk that someone might be trying to run the commands separately to get around the collision error, but I think it is unlikely.