Add default 'package_id' link meta for crates (with value equal to 'name')
Added default value for package_id attribute when encoding metadata for lib crates
Added tests for default generation of package_id meta attribute
Specify package_id for rust libs, to avoid spurious warnings
Added a FIXME with issue number to change package_id
auto merge of #10368 : tautologico/rust/default_pkgid, r=catamorphism

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 added an explicit package_id in the link attribute in all rust libs to avoid getting warnings about default package_id values when building rust.