worker/jobs: Move index-related jobs into dedicated modules

[Turbo87]

This PR splits up the worker/jobs/git module into several per-job modules, which should make the files slightly more maintainable. It also renames the parent module from git to index, since the code also deals with the sparse index, which is not git-based.

[codecov]

Codecov Report

Attention: Patch coverage is 29.28571% with 99 lines in your changes missing coverage. Please review.

Project coverage is 89.11%. Comparing base (2d33f01) to head (f693720).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
src/worker/jobs/index/normalize.rs	0.00%	63 Missing ⚠️
src/worker/jobs/index/squash.rs	0.00%	35 Missing ⚠️
src/worker/jobs/index/sync.rs	97.61%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9576   +/-   ##
=======================================
  Coverage   89.11%   89.11%           
=======================================
  Files         287      289    +2     
  Lines       29051    29051           
=======================================
  Hits        25890    25890           
  Misses       3161     3161           

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