This patch series causes package-mapping to recurse into package and bag
arguments when #:deep? #t is given.  It also recurses into gexps and
gexp inputs in search of packages to devour.  It also ensures that build
systems leave all of their implicit package arguments in the bag
arguments, ready to be found by package-mapping.  It also fixes a couple
build system errors I came across while modifying 40+ build systems and
testing a deep package transformation.

- ulfvonbelow