Fix bugs in normalize_line(), and with a 2.7x speedup

[kurtmckee]

The following script was used for performance testing:

import timeit
import isort.parse

print(
    timeit.timeit(
        'isort.parse.normalize_line("from..cimport a")',
        number=5_000_000,
        globals={"isort": isort},
    )
)

Fastest time with code in main: 15.98 seconds
Fastest time with code in this PR: 5.82 seconds

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.16%. Comparing base (3432318) to head (9aeaf27).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2381      +/-   ##
==========================================
- Coverage   99.16%   99.16%   -0.01%     
==========================================
  Files          40       40              
  Lines        3101     3100       -1     
  Branches      680      680              
==========================================
- Hits         3075     3074       -1     
  Misses         15       15              
  Partials       11       11              

[kurtmckee]

Thanks for the review, @DanielNoord! Is there anything preventing this PR (and the others you approved) from merging?

[DanielNoord]

Mostly a review from @staticdev I think