rustpkg: Always write a package_id attribute into the link metadata

For some reason, I thought it wasn't necessary to write the package_id
attribute (which rustc's filesearch checks when searching for a package)
if the package ID had a single component (like "foo") as opposed to multiple
components (like "foo/bar/quux"). This meant that
`extern mod quux = "an-awesome-library";` didn't work, even if an-awesome-library
existed in the RUST_PATH.

Fixed it.
auto merge of #9179 : catamorphism/rust/rustpkg-package-id, r=catamorphism,metajack

r? @metajack - This solves the problem you were having earlier with things like ```extern mod geom = "rust-geom";``` (or something like that).