Iterate psr-0/4 fallback dirs in composer finders

[alekitto]

Fixes #22

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.84%. Comparing base (9ae332c) to head (a6a6972).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #23   +/-   ##
=========================================
  Coverage     87.84%   87.84%           
  Complexity      384      384           
=========================================
  Files            42       42           
  Lines           913      913           
=========================================
  Hits            802      802           
  Misses          111      111           

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