Add a method for a dependency to 'push' an artifact up the dependency tree

Motivation: https://github.com/jwilm/alacritty/pull/1374#issuecomment-397724021

In the case above our binary crate depends on another binary (winpty-agent) which is built by one of it's dependants. At the moment there is no easy way for our crate to access artifacts produced in a dependencies target directory, unless it's a library linked by cargo.

alacritty <- Binary needed here
    winpty
        winpty-sys <- Binary built here

My suggestion is too add an output to the build script called 'publishes' which contains a name of an artifact produced by the build script. Setting this will make cargo copy it into any crates which require it as a dependency. In this scenario winpty-sys AND winpty would set publishes to the name of the binary. It would be copied into winpty's target directory, then into alacritty's target directory.

Start by reading Cargo's build-script output handling and how dependency target directories are populated. Trace the binary-artifact path from winpty-sys through winpty to alacritty, using the dependency tree in the issue as the entry point. Done would require an agreed design and verified propagation of the named artifact through each dependent target directory.