Eradicate use of c.pkg

[rossjones]

c.pkg is the most user occurring instance of sqlalchemy models being passed to the templates alongside pkg_dict. Persuading everyone to stop using c in extensions would take a very long time, but at least we can remove the Package model from the context passed to the template. We can then stop package_show stuffing the Package model into the context that it was passed.

This would be a breaking change, so perhaps some notice in an upcoming version and then removal in the next would be a good approach.

[rossjones]

It looks like removing the package from the context will be problematic in resource_create

    ## Get out resource_id resource from model as it will not appear in
    ## package_show until after commit
    upload.upload(context['package'].resources[-1].id,
                  uploader.get_max_resource_size())

[davidread]

General agreement - let's pick this off as we come across it.