auto merge of #9132 : catamorphism/rust/rustpkg-recursive-deps, r=catamorphism,metajack

r? @metajack ...recursive dependencies

Closes #8524
rustpkg: Install to RUST_PATH

Install to the first directory in the RUST_PATH if the user set a
RUST_PATH. In the case where RUST_PATH isn't set, the behavior
remains unchanged.

Closes #7402