"Advertise" workspace feature when dependencies are path-specified

Hi,

I just realized that in projects with multiple sub-crates, adding them as members of [workspace] really helps with "ambiguous" paths in errors and warnings, as src/lib.rs:line:column gets replaced by subcrate-path/src/lib.rs:line:column, which is awesome and will massively improve my workflow.

But I really wish I had found out sooner, or even that cargo would have let me know "Hey, you have subcrate = { path = "subcrate-path" } in your dependencies, but it isn't in your workspace members, you sure you don't wan't that to get even better error messages?"

For example with CLion, it makes AwesomeConsole able to jump to the right file rather than hesitate between each src/lib.rs in the project tree, turning cargo watch -x clippy into a great "Problems" pane.

Anyhow, it's probably very low priority, but I'm sure having cargo let you know a bit more about [workspace] would help other people too :)

Start by tracing Cargo's handling of workspace members and path-specified dependencies, using the issue's example as the desired case. The work is done when Cargo can clearly notify users that a path dependency is not listed as a workspace member, without disrupting valid projects.