refactor(main): replace deprecated pkg_resources

[tiborsimko]

Replace pkg_resources with the modern importlib.metadata API to resolve deprecation warnings.

Closes reanahub/reana-commons#502

[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.90%. Comparing base (4b6dbb7) to head (a565875).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
reana_workflow_engine_cwl/main.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##           master    #302   +/-   ##
======================================
  Coverage    0.90%   0.90%           
======================================
  Files          10      10           
  Lines         551     551           
======================================
  Hits            5       5           
  Misses        546     546           

Files with missing lines	Coverage Δ
reana_workflow_engine_cwl/main.py	0.00% <0.00%> (ø)

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