Do not stop ReadDirRecursive* on broken symlinks

[cmaglie]

Previously if a broken symlink was found the ReadDirRecursive stopped with an error.
Now the broken symlink is returned as a file.

$ tree testdata/loops/regular_3/
testdata/loops/regular_3/
|-- dir1
|   `-- file1
|-- dir2
|   |-- dir1 -> ../dir1
|   `-- file2
`-- link -> broken

ReadDirRecursive() on testdata/loops/regular_3/ now returns:

testdata/loops/regular_3/dir1
testdata/loops/regular_3/dir1/file1
testdata/loops/regular_3/dir2
testdata/loops/regular_3/dir2/dir1
testdata/loops/regular_3/dir2/dir1/file1
testdata/loops/regular_3/dir2/file2
testdata/loops/regular_3/link

(cc @matthijskooijman This is part of your PR #13 was trying to do)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.00%. Comparing base (3226a11) to head (075c5c8).
⚠️ Report is 27 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
+ Coverage   67.83%   69.00%   +1.16%     
==========================================
  Files           7        7              
  Lines         401      400       -1     
==========================================
+ Hits          272      276       +4     
+ Misses         94       92       -2     
+ Partials       35       32       -3     

Flag	Coverage Δ
unit	69.00% <100.00%> (+1.16%)	⬆️

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[alessio-perugini]

Note for us of the future: if we spot some regression, it could be related to the calling of path.IsDir() instead of path.IsDirCheck().