Track unresolvable transaction packages
Rather than free them right away, keep the list on the transaction as
we already do with add and remove lists. This is necessary because we
may be manipulating pointers the frontend needs to refer to packages,
and we are breaking our contract as stated in the alpm_add_pkg()
documentation of only freeing packages at the end of a transaction.
This fixes an issue found when refactoring the package list display
code.
Signed-off-by: Dan McGee <dan@archlinux.org>