Make commands in dependencies available to run

[tchernobog]

This approach teaches cargo run to handle also binary targets in dependencies. In turn, it allows using cargo as a shim
when needing to invoke a specific package version from the set of dependencies, without resorting to install a user-wide
version of a binary.

Users of e.g. mdbook should rejoice.

Note that this is my first contribution to cargo. I am not familiar yet on how is the best way to do things.
Also, I haven't adjusted any documentation yet, I wanted first to see if my idea is sound or not.

If approved, fixes #2267.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Eh2406 (or someone else) soon.

Please see the contribution instructions for more information.

[alexcrichton]

Thanks for the PR, but this is a pretty large feature to add to Cargo, so I don't think that a PR is necessarily the best starting point. If you're interested in pursuing this I think the first thing to do is to write up a design of what this would look like and get consensus around that first. This may also end up requiring an RFC for broader feedback as well.

[tchernobog]

Thanks for the PR, but this is a pretty large feature to add to Cargo, so I don't think that a PR is necessarily the best starting point. If you're interested in pursuing this I think the first thing to do is to write up a design of what this would look like and get consensus around that first. This may also end up requiring an RFC for broader feedback as well.

Thanks @alexcrichton, I opened rust-lang/rfcs#3168 as per your suggestion.

[Eh2406]

I will close for now. We can reopen when the RFC is discussed.