Revert "lib/repo_pkgdeps.c: check for wrong dependencies earlier."

This reverts commit eed9c893757b0b90d4118656b1055982efbae441.
xbps_repo_get_pkg_revdeps: ignore invalid repos.
xbps_rpool_foreach: ignore invalid repos.
Introduce xbps_{array,pkgdb}_foreach_cb_multi() and use it where appropiate.

In some tasks the single threaded implementation outperms the multithreaded
one. Use it where it really makes a difference. The _multi() routines do not
spawn any thread if _SC_NPROCESSORS_ONLN == 1.

Bump XBPS_API_VERSION.