Fix importlib.resources usage in simulation

[robtaylor]

SOURCE_DIR  = importlib.resources.files("my_design") / "sim"
COMMON_DIR  = importlib.resources.files("common") / "sim"
SOURCE_DIR  = importlib.resources.files("mcu_soc") / "design/sim"
``
Will fail on non-filesystem `importlib` backends, should use `importlib.resources.as_file`, which can return a directory. Needs some work to make sure builds are done within the context manager it returns.