xbps-src: check for existing packages before syncing chroot repodata

This significantly speeds up the package building process for build
scripts etc. that run './xbps-src pkg ...' on a long list of packages
that may already have up-to-date local packages built when configured
with XBPS_PRESERVE_PKGS set.

xbps-src

@@ -806,6 +806,7 @@ case "$XBPS_TARGET" in
         if [ -n "$CHROOT_READY" -a -z "$IN_CHROOT" ]; then
             chroot_handler $XBPS_TARGET $XBPS_TARGET_PKG
         else
+            check_existing_pkg
             chroot_sync_repodata
             # prevent update_base_chroot from removing the builddir/destdir
             update_base_chroot keep-all-force