Raise an error if SERVICES does not define a valid shepherd service graph, for instance if a service requires a nonexistent service, or if more than one service uses a given name.

These are constraints that shepherd's 'register-service' verifies but we'd better verify them here statically than wait until PID 1 halts with an assertion failure.

Return a procedure that, when passed a symbol, return the item among SERVICES that provides this symbol. PROVISION must be a one-argument procedure that takes a service and returns the list of symbols it provides.

Return two values: the subset of LIVE (a list of <live-service>) that needs to be unloaded, and the subset of LIVE that needs to be restarted to complete their upgrade.