Don't fallback to Imath from src/deps

[jlskuz]

If the user explicitely requests to use find_package to use the system Imath it seems wrong to fallback to the delivered Imath

As discussed in #1859

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.72%. Comparing base (c0e97b0) to head (f0bbf7a).
Report is 55 commits behind head on main.

❌ Your changes status has failed because you have indirect coverage changes. Learn more about Unexpected Coverage Changes and reasons for indirect coverage changes.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1864      +/-   ##
==========================================
+ Coverage   84.11%   84.72%   +0.61%     
==========================================
  Files         198      177      -21     
  Lines       22241    12809    -9432     
  Branches     4687     1191    -3496     
==========================================
- Hits        18709    10853    -7856     
+ Misses       2610     1773     -837     
+ Partials      922      183     -739     

Flag	Coverage Δ
py-unittests	84.72% <ø> (+0.61%)	⬆️

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see 131 files with indirect coverage changes

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[darbyjohnston]

Looks good, thanks for following up on this.