Add support for .prekignore to ignore directories from project discovery

[j178]

Suggested by @potiuk from #626 (comment)

Closes #750
Closes #751

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.39%. Comparing base (5d32b2f) to head (a474cb6).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #826      +/-   ##
==========================================
+ Coverage   89.38%   89.39%   +0.01%     
==========================================
  Files          59       59              
  Lines       10958    10959       +1     
==========================================
+ Hits         9795     9797       +2     
+ Misses       1163     1162       -1     

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