👌 Improve warning for needextract incompatibility with external needs

[chrisjsewell]

closes #677

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 87.90%. Comparing base (4e10030) to head (2d64c3f).
Report is 72 commits behind head on master.

Files with missing lines	Patch %	Lines
sphinx_needs/directives/needextract.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1325      +/-   ##
==========================================
+ Coverage   86.87%   87.90%   +1.02%     
==========================================
  Files          56       60       +4     
  Lines        6532     7009     +477     
==========================================
+ Hits         5675     6161     +486     
+ Misses        857      848       -9     

Flag	Coverage Δ
pytests	87.90% <0.00%> (+1.02%)	⬆️

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