Remove superflous subfolders of src

[Flamefire]

The shorter paths make the sources easier to navigate.

Basically revert of #99 / f44b3bb after the underlying issue is resolved.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.87%. Comparing base (abd3b98) to head (59fa713).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #255      +/-   ##
===========================================
+ Coverage    95.72%   95.87%   +0.15%     
===========================================
  Files          117      117              
  Lines        10386    10406      +20     
===========================================
+ Hits          9942     9977      +35     
+ Misses         444      429      -15     

Files with missing lines	Coverage Δ
src/encoding/codepage.cpp	100.00% <ø> (ø)
src/encoding/iconv_converter.hpp	94.64% <ø> (ø)
src/encoding/uconv_converter.hpp	100.00% <ø> (ø)
src/encoding/wconv_converter.hpp	91.61% <ø> (ø)
src/icu/boundary.cpp	93.81% <ø> (ø)
src/icu/cdata.hpp	100.00% <ø> (ø)
src/icu/codecvt.cpp	84.09% <ø> (ø)
src/icu/collator.cpp	94.20% <ø> (ø)
src/icu/conversion.cpp	93.33% <ø> (ø)
src/icu/date_time.cpp	92.85% <ø> (ø)
... and 57 more

... and 4 files with indirect coverage changes

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