auto merge of #10110 : catamorphism/rust/rustpkg-dependency-build-dir, r=metajack

r? @metajack When invoked with the --rust-path-hack flag, rustpkg was correctly building
the package into the default workspace (and not into the build/ subdirectory of the
parent directory of the source directory), but not correctly putting the output
for any dependencies into the default workspace as well.

Spotted by Jack.

diff --cc src/librustpkg/rustpkg.rs
Simple merge

diff --cc src/librustpkg/tests.rs
Simple merge

diff --cc src/librustpkg/util.rs
Simple merge