libalpm now exports type alpm_pkgfrom_t in alpm.h, which may be used
by frontends.

Pacman now uses alpm_pkgfrom_t instead of replicating that type (pkg_from
as was in src/pacman/package.h)

Updated API change in README.

Signed-off-by: Diogo Sousa <diogogsousa@gmail.com>
Signed-off-by: Dan McGee <dan@archlinux.org>