Move _cache_transform to its own file in qml.workflow

[andrijapau]

Context:

Introduces a new file _cache_transform.py in qml.workflow to house the private _cache_transform helper. Previously it lived in execution.py despite having its own unit test file workflow/test_cache_transform.py.

Also, update file name: resolve_diff_method.py → _resolve_diff_method.py as I should have done earlier. 😄

Benefits: Code consistency and clean-up

[github-actions]

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[JerryChen97]

LGTM!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.45%. Comparing base (76f9d7c) to head (183635d).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6624   +/-   ##
=======================================
  Coverage   99.45%   99.45%           
=======================================
  Files         451      452    +1     
  Lines       42293    42323   +30     
=======================================
+ Hits        42062    42092   +30     
  Misses        231      231           

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