Add a default value for the 'package_id' meta attribute for library crates

This Fixes #10265 and paves the way for fixing #9543. It works by adding a 'package_id' attribute by default for library crates that don't specify it. This is necessary to use the 'extern mod foo = "bar"' form instead of 'extern mod foo(name="bar") (as per #9543), because the former adds a required package_id when trying to link with the bar crate. I added a simple test to ensure that the default package_id value is being generated, and also changed the link attribute in all rust libs to avoid getting warnings about default package_id values when building rust.