topdown: Do not memoize partial rules if dependencies are unknown

The evaluator memoizes partial rules to avoid recomputing key/value
pairs unnecessarily. However, the cache only supports term values--not
arbitrary expressions--so it cannot be used to cache partial
evaluation. This commit fixes the evaluator to skip memoization if
partial evaluation is enabled and the ref depends on unknowns.

Fixes #2455

Signed-off-by: Torin Sandall <torinsandall@gmail.com>