Extract index module

[Turbo87]

This module contains the glue code between our database and the index files and is used by the corresponding background jobs to generate the index files.

The main purpose of this change is to pull the index-related code out of the models/krate module. I could've pulled it into the background job module, but that felt a bit dirty compared to having a dedicated glue code module.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.69%. Comparing base (2eec9bb) to head (43c233b).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9651   +/-   ##
=======================================
  Coverage   88.69%   88.69%           
=======================================
  Files         288      288           
  Lines       29368    29371    +3     
=======================================
+ Hits        26048    26051    +3     
  Misses       3320     3320           

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