Make <Arg> element for <Invoke> consider not only the literals

[evshvarov]

Currently expects literals, but it is not convenient - e.g., it doesn't allow expressions to be made within module.xml.
Could we introduce, e.g., element (meaning ArgExpression), where we can use strings and combine with IPM variables like ${libdir}, ${ipmdir}, module parameters, etc?
It will also be consistent with all other elements of module.xml.
Thank you.