Replace relative path determination from proprietary to std

[zeek0x]

I don't have a windows development environment, so I can't confirm that this will work as before, but I don't think replacing it with std will cause any problems.

[codecov]

Codecov Report

Base: 92.38% // Head: 92.54% // Increases project coverage by +0.15% 🎉

Coverage data is based on head (b760cd8) compared to base (3c4d3b3).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2345      +/-   ##
==========================================
+ Coverage   92.38%   92.54%   +0.15%     
==========================================
  Files          37       37              
  Lines        4454     4441      -13     
==========================================
- Hits         4115     4110       -5     
+ Misses        247      241       -6     
+ Partials       92       90       -2     

Impacted Files	Coverage Δ
echo_fs.go	92.50% <100.00%> (+7.55%)	⬆️
middleware/jwt.go	92.98% <100.00%> (ø)

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[aldas]

On windows isAbs does more work (checks special names like 'COM1' CON etc) but in that sense it is more correct and as we this function is not called during request doing more work is ok.