refactor(python): Refactor utils/convert.py module

[stinodego]

Some renaming/reorganization. Also some very minor performance optimizations but none really worth mentioning. No functional changes.

[codecov]

Codecov Report

Attention: Patch coverage is 85.52632% with 22 lines in your changes are missing coverage. Please review.

Project coverage is 81.06%. Comparing base (b4ed30b) to head (eb53514).
Report is 3 commits behind head on main.

❗ Current head eb53514 differs from pull request most recent head 78e65ee. Consider uploading reports for the commit 78e65ee to get more accurate results

Files	Patch %	Lines
py-polars/polars/utils/convert.py	84.52%	11 Missing and 2 partials ⚠️
py-polars/polars/datatypes/convert.py	0.00%	4 Missing ⚠️
crates/polars-plan/src/logical_plan/pyarrow.rs	50.00%	3 Missing ⚠️
py-polars/polars/series/datetime.py	60.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #14739   +/-   ##
=======================================
  Coverage   81.06%   81.06%           
=======================================
  Files        1322     1322           
  Lines      171356   171355    -1     
  Branches     2461     2456    -5     
=======================================
+ Hits       138904   138907    +3     
+ Misses      31981    31979    -2     
+ Partials      471      469    -2     

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