lib/transaction_sortdeps.c: there's no need to use TAILQ_FOREACH_SAFE.
rpool: switch "struct xbps_repo" to be part of the rpool simpleq.

The previous internal "struct rpool" was an extra structure that
can be avoided by just using "struct xbps_repo" directly.

This makes rpool use (at least) 4KB less per repository and 1
extra allocation.