Move the ports INDEX fetch functionality from webupdate.wrapper
to webupdate to fix the weekly clean build failures.

When fullbuild-clean.flag exists, all of the trees are removed
and checked out cleanly. This happens after the INDEX-${INDEXNUM}
is fetched, so when the build enters the htdocs/ports/ directory,
the INDEX-${INDEXNUM} no longer exists.

Approved by: doceng (implicit)
Sponsored by: The FreeBSD Foundation