Move class exclusion checking from KeepAgentPromise to ExpandPromise

Promise.classes cannot contain expandable variables, so we may
exclude a promise early without evaluating it fully.
Move VarClassExcluded from KeepAgentPromise to ExpandPromiseAndDo
Move MissingDependencies from KeepAgentPromises to ExpandPromiseAndDo

Additionally, remove namespace parsing for depends_on.