rustpkg names executables incorrectly

[catamorphism]

@bjz pointed out the following example with glfw:

https://gist.github.com/bjz/8aac0e5338f50b88e31c

The repository contains several subdirectories under examples/, each containing a main.rs, but rustpkg only leaves one executable (named examples) under build/. It should be creating a subdirectory of build/ for each of the examples/ subdirectories.

[catamorphism]

This is strictly part of #7240, I think. Closing.