packagedb: don't keep/pass around manifests_dir

Store 'dirname' as the toplevel packagedb directory instead of storing
the manifests_dir.  Pass dirname into the individual package objects and
let them do the path pasting for themselves.

Also: hold off on actually creating the manifests directory until we
actually go to write a file there.

This removes all logic about the location of the 'manifests' directory
from the toplevel PackageDB object.

https://bugzilla.gnome.org/show_bug.cgi?id=743127