[Dexter] Set up ComInterface module to be imported correctly (#111850)
Fixes issue added by: https://github.com/llvm/llvm-project/pull/111833
Following the previous commit that changed how Dexter imports modules,
the ComInterface module import became broken. This is because it had a
different directory structure to other modules, where we want to import
single file rather than a dir containing a __init__.py. For this case,
an optional extra arg has been added to load_module allowing a filename
to be specified, letting us import ComInterface.py directly and fixing
the issue.