Add a way to add a sub-module without connecting it to any ports of the containing module

### Motivation

Sometimes a "top" level module is purely for integration purposes (e.g. for testbench purposes, or early in development), and so it has no external top-level ports. In this case, there should be a way to add a sub-module explicitly as being within a given module even though it can't be found automatically by tracing signals.

### Desired solution

Add a function, maybe like `addSubModule`. This should not be a general requirement or standard expectation, and only used in specific scenarios.

Other modules connected to these special sub-modules should be considered within the same context of the containing module, so this adds some additional complexity.

### Alternatives considered

Just require at least one top-level port to hook up to at least one sub-module (the current workaround).

### Additional details

_No response_