Handle Invalid Symlinks During Scans

[6d7a]

Context

When calling ggshield secret scan ./dir-containing-invalid-symlink on a directory that contains an invalid symlink, the command fails. This PR fixes it.

Closes #1144.

What has been done

• Catch exceptions caused by invalid symlinks during scans and log a warning.
• Add test case for invalid symlinks.

Validation

• create a test target for the symlink: touch /tmp/testfile
• create a symlink to the test file: ln -s /tmp/testfile
• delete the symlink target: rm /tmp/testfile
• run ggshield secret scan path --recursive .
• confirm that ggshield logs the skipped file it did not find

PR check list

• As much as possible, the changes include tests (unit and/or functional)
• If the changes affect the end user (new feature, behavior change, bug fix) then the PR has a changelog entry (see doc/dev/getting-started.md). If the changes do not affect the end user, then the skip-changelog label has been added to the PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.96%. Comparing base (705db4e) to head (1776b65).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1147      +/-   ##
==========================================
+ Coverage   91.94%   91.96%   +0.02%     
==========================================
  Files         144      144              
  Lines        6196     6199       +3     
==========================================
+ Hits         5697     5701       +4     
+ Misses        499      498       -1     

Flag	Coverage Δ
unittests	91.96% <100.00%> (+0.02%)	⬆️

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[agateau-gg]

Looks good, thanks!