Add --force-separate-as-imports option to keep non-aliased imports grouped together

[n0gu-furiosa]

Introduces --force-separate-as-imports, which separates the non-aliased import group from the aliased import group.

This is a proposed minimal change for #2455. The design for this issue is not decided yet, so this PR is a demo and does not include documentation or test updates. Once the design is accepted, I will work on those too.

File1.py:

from module import (
    AAAAAAAAAAAAAAAAAAAAAAAAAA,
    BBBBBBBBBBBBBBBBBBBBBBBBBB,
    CCCCCCCCCCCCCCCCCCCCCCCCCC,
)
from module import DDDDDDDDDDDDDDDDDDDDDDDDDD as d
from module import (
    EEEEEEEEEEEEEEEEEEEEEEEEEE,
    FFFFFFFFFFFFFFFFFFFFFFFFFF,
    GGGGGGGGGGGGGGGGGGGGGGGGGG,
)

Without the option (which behaves the same as main, no breaking changes)

$ isort file1.py -d --fss
from module import (AAAAAAAAAAAAAAAAAAAAAAAAAA, BBBBBBBBBBBBBBBBBBBBBBBBBB,
                    CCCCCCCCCCCCCCCCCCCCCCCCCC)
from module import (EEEEEEEEEEEEEEEEEEEEEEEEEE, FFFFFFFFFFFFFFFFFFFFFFFFFF,
                    GGGGGGGGGGGGGGGGGGGGGGGGGG)
from module import DDDDDDDDDDDDDDDDDDDDDDDDDD as d

With the option:

$ isort file1.py -d --fss --fsai
from module import (AAAAAAAAAAAAAAAAAAAAAAAAAA, BBBBBBBBBBBBBBBBBBBBBBBBBB,
                    CCCCCCCCCCCCCCCCCCCCCCCCCC, EEEEEEEEEEEEEEEEEEEEEEEEEE,
                    FFFFFFFFFFFFFFFFFFFFFFFFFF, GGGGGGGGGGGGGGGGGGGGGGGGGG)
from module import DDDDDDDDDDDDDDDDDDDDDDDDDD as d

[n0gu-furiosa]

Hi @DanielNoord, could you take a look and share your thoughts on this PR when you have a moment?

[DanielNoord]

See my reply in #2455

I think we can resolve this as a breaking change to the style without adding another option.

[DanielNoord]

To be superseded by #2612