auto merge of #8176 : catamorphism/rust/rustpkg-extern-mod, r=catamorphism

r? @graydon Also, notably, make rustpkgtest depend on the rustpkg executable (otherwise, tests that shell out to rustpgk might run when rustpkg doesn't exist).

diff --cc mk/tests.mk
Simple merge

diff --cc src/librustc/back/rpath.rs
Simple merge

diff --cc src/librustc/metadata/filesearch.rs
Simple merge

diff --cc src/librustc/middle/resolve.rs
Simple merge

diff --cc src/libsyntax/parse/parser.rs
Simple merge