MAINT: Replace ported sources with shim imports and deprecation warnings

[oesteban]

Resolves: #787.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.49%. Comparing base (c2b8e0f) to head (f1adf7e).
⚠️ Report is 418 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #789      +/-   ##
==========================================
- Coverage   63.39%   60.49%   -2.90%     
==========================================
  Files          49       42       -7     
  Lines        5963     4863    -1100     
  Branches     1170      943     -227     
==========================================
- Hits         3780     2942     -838     
+ Misses       2001     1808     -193     
+ Partials      182      113      -69     

Flag	Coverage Δ
reportlettests	∅ <ø> (∅)
unittests	55.01% <ø> (+4.35%)	⬆️

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[effigies]

I would specifically not like to depend on nireports. I would just deprecate and not shim. We stop fixing things here, and in a year or so we just remove.