Generate real requirements.txt instead of versions.txt

[wuhu]

• creates a requirements.txt instead of a versions txt
• this uses importlib_metadata.packages_distributions to find the correct name and version
• if a package cannot be found (for example if it's not installed), an exception is raised, this can be suppressed using save(..., strict_requirements=False)

[codecov-commenter]

Codecov Report

Merging #425 (869cc1f) into main (232ff3d) will increase coverage by 0.07%. The diff coverage is 97.36%.

@@            Coverage Diff             @@
##             main     #425      +/-   ##
==========================================
+ Coverage   88.18%   88.26%   +0.07%     
==========================================
  Files          15       15              
  Lines        3065     3085      +20     
==========================================
+ Hits         2703     2723      +20     
  Misses        362      362              

Impacted Files	Coverage Δ
padl/dumptools/packagefinder.py	96.55% <96.00%> (+1.42%)	:arrow_up:
padl/transforms.py	91.35% <100.00%> (+0.01%)	:arrow_up:

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