Split basic import statements with multiple imported names

[amyreese]

Before sorting and merging imports within a block, this looks for basic
(non-from) imports with more than one imported name, and then splits
that import into a separate import for each name. Comments are
duplicated from the original import to all split imports.

Fixes #140

Docs preview: https://usort--141.org.readthedocs.build/en/141/

[amyreese]

• Still needs documentation.

[amyreese]

Some notes from discussion:

• Consider if old behavior is potentially unstable in 1.0.1, (eg, import thirdparty, os gets changed to import os, thirdparty). If unstable, submit separate bug fix for a 1.0.2 release.
• Look for test cases that might fail if evolve() isn't used.
• Add test case covering combined split+merge with comments.
• Determine if new behavior is fully "back compatible", ie, will 1.0 or 0.6.4 preserve changes made when splitting.
• Consider if this warrants a 2.0 version bump

[amyreese]

The existing test case already fails if evolve() is not used:

AssertionError: µsort result did not match expected value:

Before:
-------

import sys, os, fancy, time, re
import math, usort, bisect

Expected:
---------

import bisect
import math
import os
import re
import sys
import time

import fancy
import usort

Result:
-------

import bisect
import math

import os

import re

import sys

import time

import fancy
import usort

Because fancy was part of the same group, but a different category, the blank line meant to separate the categories ends up applying to the stdlib imports too.