Cached PhutilClassMapQuery which can perform lookups for single classes efficiently.

Some class trees (like Conduit methods and PHID types) contain a huge number of classes but are frequently accessed by looking for a specific class by a known identifier (like a Conduit method name or a PHID type constant).

Loading the entire class map for these cases has a small but measurable performance cost. Instead, we can build a cache from each Conduit method name to just the class required to serve that request. This means that we load fewer classes and have less overhead to execute API calls.