refactor(cache): move IQBCache to cache.py

[bassosimone]

No functional change, just avoiding to implement lots of stuff inside __init__.py because it's not super idiomatic IIUC.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.51%. Comparing base (054b64f) to head (2440a5b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #67      +/-   ##
==========================================
+ Coverage   82.50%   82.51%   +0.01%     
==========================================
  Files          27       28       +1     
  Lines        2269     2271       +2     
==========================================
+ Hits         1872     1874       +2     
  Misses        397      397              

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