it was previously not possible to use mksource() in a split-package
the mksource temporary PKGBUILD over-rides the package() function;
but conflicting package_*() functions were left in tact
libremakepkg will refuse to build a blacklisted package,
if no libre replacement had been published before the next repo sync
this change warns instead, and prompts for confirmation
see note in src/chroot-tools/hooks-check.sh::check_pkgbuild_nonfree()
