Separate interfaceGenerator from specGenerators

The InterfaceGenerator is special and its result is not included in meta. So separate it in the code, too.
Is it ok that there are no error handlings here?

Errors that occur in spec.Collect have been ignored: https://github.com/mackerelio/mackerel-agent/blob/split-spec-interface-generator/spec/spec.go#L17-L23