Use std::filesystem in filesys_tools

[RainerKuemmerle]

• Refactor dl_wrapper to use updated filesys_tools
• Use unordered_set in dl_wrapper

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (b03c874) 50.88% compared to head (85695ac) 50.87%.

Files	Patch %	Lines
g2o/stuff/filesys_tools.cpp	90.24%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #748      +/-   ##
==========================================
- Coverage   50.88%   50.87%   -0.01%     
==========================================
  Files         271      271              
  Lines       10966    10972       +6     
==========================================
+ Hits         5580     5582       +2     
- Misses       5386     5390       +4     

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