We are currently mapping input/output statements differently between 'action' and 'rpc'. While 'action' allocates a sub-package and uses plain Input/Ouput, 'rpc' does not allocate a subpackage and drops the Foo (the RPC as of MDSAL-773), FooInput and FooOutput in the same package.

While the former matches instantiated tree semantics, the latter is actually more friendly to use, as the distinct names can easily be imported and worked with.

Change the way action's input/output is generated to match the way RPCs do it.