Use find_package arguments to mandate NumPy

[knoepfel]

This change allows a CMake-only replacement to introducing numpy build dependencies.

Also included:

• The automatic addition of the Python home if the VIRTUAL_ENV environment variable is available
• The automatic addition of any paths from CMAKE_PREFIX_PATH to sys.path (required due to a Spack infelicity)
• The replacement of one find_package(Python3 ...) command with find_package(Python ...) as Python2 is not supported.

[knoepfel]

@phlexbot format

[github-actions]

No automatic clang-format fixes were necessary.

[github-actions]

Automatic cmake-format fixes pushed (commit e69700a).
⚠️ Note: Some issues may require manual review and fixing.

[knoepfel]

@phlexbot format

[github-actions]

No automatic clang-format fixes were necessary.

[github-actions]

Automatic cmake-format fixes pushed (commit dee425d).
⚠️ Note: Some issues may require manual review and fixing.

[codecov]

Codecov Report

❌ Patch coverage is 65.51724% with 10 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
plugins/python/src/pymodule.cpp	62.96%	2 Missing and 8 partials ⚠️

❌ Your patch status has failed because the patch coverage (65.51%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.
❌ Your project status has failed because the head coverage (75.67%) is below the target coverage (80.00%). You can increase the head coverage or adjust the target coverage.

@@            Coverage Diff             @@
##             main     #219      +/-   ##
==========================================
- Coverage   76.66%   75.67%   -0.99%     
==========================================
  Files         124      124              
  Lines        2725     2902     +177     
  Branches      475      507      +32     
==========================================
+ Hits         2089     2196     +107     
- Misses        442      487      +45     
- Partials      194      219      +25     

Flag	Coverage Δ
unittests	75.67% <65.51%> (-0.99%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
plugins/python/src/modulewrap.cpp	46.03% <100.00%> (+5.25%)	⬆️
plugins/python/src/pymodule.cpp	60.00% <62.96%> (-4.59%)	⬇️

... and 3 files with indirect coverage changes

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