TDD testcases for allowing `cargo run` to be invoked
on dependencies of the workspace members via their
corresponding pkgid.

Relates to rust-lang#2267, rust-lang#872.

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.

Fixes rust-lang#2267.

Relates to rust-lang#2267.