Changed filename field to be explicit

[RobertoRoos]

Resolves #87

• :filename: is resolved only to current directory #87

This would supersede #73

• the extension should primarly resolve the srcdir. #73

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.15%. Comparing base (4564fdd) to head (bbe703c).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
sphinxarg/ext.py	55.55%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #88      +/-   ##
==========================================
- Coverage   51.94%   50.15%   -1.79%     
==========================================
  Files          18       18              
  Lines        1286     1288       +2     
  Branches      261      262       +1     
==========================================
- Hits          668      646      -22     
- Misses        575      598      +23     
- Partials       43       44       +1     

Flag	Coverage Δ
unittests	49.92% <71.42%> (-1.79%)	⬇️

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