setup: make "tarball" a new default PKG_TYPE

Make "tarball" a new default PKG_TYPE.
setup: introduce PKG_DATE

Introduce PKG_DATE. It should document the date when a package's
upstream status (URL, version) was last checked, i.e. similar to
a retrieval date you often see in LaTeX URL citations etc.
setup: display hash correctly when listing packages

Display hash correctly when listing packages.