KeyError while sorting duplicate imports

[zsol]

On this file:

from a import x
from b import y
from b import y
from c import x

usort fails with

Traceback (most recent call last):
  File "usort/cli.py", line 80, in list_imports
    blocks = sorter.sortable_blocks(mod.body)
  File "usort/sorting.py", line 192, in sortable_blocks
    current = self.split_inplace(current, overlap)
  File "usort/sorting.py", line 169, in split_inplace
    new.imported_names[key] = block.imported_names.pop(key)
KeyError: 'y'

[zsol]

The problem is that this code assumes that the newly split block doesn't have shadowing. This assumption isn't satisified by the logic here.

We either need to fix name_overlap() to provide this assumption or split_inplace() to not need it.