rebase: refer to checkunresolved by its new name

checkunresolved: move to new package to help avoid import cycles This will allow localrepo to stop using cmdutil, which should avoid some future import cycles. There's room for an adventurous soul to delve deeper into merge.py and figure out how to disentangle more of it - it appears to be a nexus of cycle problems. Some of it might be able to move into this new mergeutil package.

cmdutil: mark dirstateguard as deprecated I sincerely doubt this is used in external code, as grepping the extensions I keep locally (including Facebook's hgexperimental and evolve) indicate nobody outside of core uses this. As such, I'd also welcome just dropping this name forward entirely.