Auto merge of #5126 - gilescope:cycle-error-message, r=alexcrichton

Error message for package "depends on itself" lists the packages in the cycle.

I got a cycle while trying to build someone else's code and cargo's error message didn't point me in the right direction, just mentioned there was a cycle. I thought we could be a bit more helpful.

Don't know what you think of {:#?} as the display but it seemed minimal code so I thought I'd start with that. I could compress the output to one package per line if preferred.

diff --cc src/cargo/core/resolver/mod.rs
Simple merge

diff --cc src/cargo/lib.rs
Simple merge

diff --cc tests/testsuite/build.rs
Simple merge