Fix relative symlinks within a mounted filesystem

[Schamper]

Fixes #831

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 75.51%. Comparing base (d5c7315) to head (e5e4f15).
Report is 1 commits behind head on main.

Files	Patch %	Lines
dissect/target/filesystems/itunes.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #832   +/-   ##
=======================================
  Coverage   75.51%   75.51%           
=======================================
  Files         305      305           
  Lines       26343    26343           
=======================================
  Hits        19893    19893           
  Misses       6450     6450           

Flag	Coverage Δ
unittests	75.51% <87.50%> (ø)

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