Files globs

Here is my attempt at enabling `files: [glob*]` entries in build stages. I think the globs should be expanded as early on as possible, inside `Package.load()`. The loading and postprocessing was mashed into `spec/package.py` and I found it a bit hard to follow, so I started by refactoring everything that doesn't belong to `PackageSpec` to the new `PackageLoader`. 

There is a bit of a question how to match globs, surely you don't want `*` to match all of hashstack... So I decided to allow `files:` entries only in directory-packages (as opposed to standalone yaml files) and match the glob relative to this package directory.