externals are excluding dependencies withth same prefix as the source module

[dsun-cp]

Here is a test case

def test_relative_imports_same_prefix(capsys):
    files = """
        foo:
            - __init__.py
            - a.py: |
                from foobar import b
        foobar:
            - __init__.py
            - b.py
    """
    with create_files(files) as workdir:
        assert simpledeps('foo') == {
            'foobar -> foo.a',
            'foobar.b -> foo.a'
        }
        pydeps(fname='foo', externals=True)
        io = capsys.readouterr()
        assert ast.literal_eval(io.out) == ['foobar']

At the last line, the current bug is causing it returning []