Fix key in cfg semantics to handle edge cases with '???' and with interpolations that cannot be resolved.

[omry]

Change semantics of key in cfg to be reasonable when key points to '???', or
is an interpolation that points to '???', or an interpolation that cannot be resolved.

desired behavior:

@pytest.mark.parametrize("conf,key,expected", [
    ({"a": 1, "b": {}}, "a", True),
    ({"a": 1, "b": {}}, "b", True),
    ({"a": 1, "b": {}}, "c", False),
    ({"a": 1, "b": "${a}"}, "b", True),
    ({"a": 1, "b": "???"}, "b", False),
    ({"a": 1, "b": "???", "c": "${b}"}, "c", False),
    ({"a": 1, "b": "${not_found}"}, "b", False),
])
def test_in_dict(conf, key, expected):
    conf = OmegaConf.create(conf)
    ret = key in conf
    assert ret == expected