depth parameter

[seddonym]

It would be useful to be able to specify the module depth of certain results.

For example:

find_upstream_modules('mypackage.foo') == {
    'mypackage.bar.one',
    'mypackage.baz.two.green',
    'mypackage.baz.two.blue',
}

# Possibly this should error as it's not very useful?
find_upstream_modules('mypackage.foo', depth=1) == {
    'mypackage',
}

find_upstream_modules('mypackage.foo', depth=2) == {
    'mypackage.bar',
    'mypackage.baz',
}

find_upstream_modules('mypackage.foo', depth=3) == {
    'mypackage.bar.one',
    'mypackage.baz.two.green',
    'mypackage.baz.two.blue',
}