Remove usage of import_module in idom.run

[rmorshea]

Current Situation

There was a bug in idom.run because we failed to update an import after a name change.

Proposed Actions

We used import_module("idom.backend.default") presumably because it was syntactically convenient to write this. Instead we should just do an actual import:

if not implementation:  # pragma: no cover
    from idom.backend import default as implementation