build(ExternalLibs/mdspan): prefer find_package over fetchcontent

[pratikpc]

Closes #652

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.35%. Comparing base (a9ce6c3) to head (e78a802).
⚠️ Report is 2 commits behind head on mainline.

Additional details and impacted files

@@            Coverage Diff            @@
##           mainline     #653   +/-   ##
=========================================
  Coverage     84.35%   84.35%           
=========================================
  Files            75       75           
  Lines          6653     6653           
  Branches        543      543           
=========================================
  Hits           5612     5612           
  Misses         1041     1041           

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