Another fix for cross-compiled build scripts

Previously the host/target requirement for packages was not correctly calculated
as dependency edges to build dependencies weren't traversed by accident.
auto merge of #815 : alexcrichton/cargo/another-fix-oh-by-wait-i-verified-this-one-its-ok, r=alexcrichton

Previously the host/target requirement for packages was not correctly calculated
as dependency edges to build dependencies weren't traversed by accident.