[DOP-14063] Drop onetl._internal module

[dolfinus]

Change Summary

Move functions from onetl._internal module to submodules of onetl._util. This module is not a part of public interface, so this is not a breaking change, and there is no item in the changelog.

Related issue number

Checklist

• Commit message and PR title is comprehensive
• Keep the change as small as possible
• Unit and integration tests for the changes exist
• Tests pass on CI and coverage does not decrease
• Documentation reflects the changes where applicable
• docs/changelog/next_release/<pull request or issue id>.<change type>.rst file added describing change
  (see CONTRIBUTING.rst for details.)
• My PR is ready to review.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.88%. Comparing base (f50f008) to head (85c792a).
Report is 58 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #290      +/-   ##
===========================================
+ Coverage    94.87%   94.88%   +0.01%     
===========================================
  Files          210      210              
  Lines         8207     8188      -19     
  Branches      1413     1410       -3     
===========================================
- Hits          7786     7769      -17     
+ Misses         298      297       -1     
+ Partials       123      122       -1     

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