feat(target): add use_calling_fname kwarg to constructor

[oldskoolfan]

• useful when calling externals() directly and passing a target class that represents a nested source file.
• the flag is used in a new get_src_fname() method on the target class, which is called by DummyModule to get text content

[oldskoolfan]

resolves #232

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.71%. Comparing base (7e742cb) to head (e31b6fa).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #233      +/-   ##
==========================================
+ Coverage   73.56%   73.71%   +0.15%     
==========================================
  Files          19       19              
  Lines        1910     1914       +4     
  Branches      356      356              
==========================================
+ Hits         1405     1411       +6     
+ Misses        398      397       -1     
+ Partials      107      106       -1     

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[thebjorn]

Thank you for the PR. It will be on PyPI as v3.0.1 as soon as the pipeline finishes.