Use pathlib.Path() in catalog frontend; improve test coverage

[akx]

This is related to #1202 – the deeply nested os.path.join() would have formatted quite poorly, so it felt like a good idea to unnest it – and then I noticed the refactored codepaths weren't tested...

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 91.66667% with 3 lines in your changes missing coverage. Please review.

Project coverage is 91.98%. Comparing base (d7a7589) to head (f2b4e8e). Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
babel/messages/frontend.py	91.66%	3 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1204      +/-   ##
==========================================
+ Coverage   91.71%   91.98%   +0.26%     
==========================================
  Files          27       27              
  Lines        4685     4691       +6     
==========================================
+ Hits         4297     4315      +18     
+ Misses        388      376      -12     

Flag	Coverage Δ
macos-14-3.10	91.02% <91.66%> (+0.26%)	:arrow_up:
macos-14-3.11	90.96% <91.66%> (+0.26%)	:arrow_up:
macos-14-3.12	91.17% <91.66%> (+0.26%)	:arrow_up:
macos-14-3.13	91.17% <91.66%> (+0.26%)	:arrow_up:
macos-14-3.8	90.89% <91.66%> (+0.26%)	:arrow_up:
macos-14-3.9	90.95% <91.66%> (+0.26%)	:arrow_up:
macos-14-pypy3.10	91.02% <91.66%> (+0.26%)	:arrow_up:
ubuntu-24.04-3.10	91.04% <91.66%> (+0.26%)	:arrow_up:
ubuntu-24.04-3.11	90.98% <91.66%> (+0.26%)	:arrow_up:
ubuntu-24.04-3.12	91.19% <91.66%> (+0.26%)	:arrow_up:
ubuntu-24.04-3.13	91.19% <91.66%> (+0.26%)	:arrow_up:
ubuntu-24.04-3.8	90.91% <91.66%> (+0.26%)	:arrow_up:
ubuntu-24.04-3.9	90.97% <91.66%> (+0.26%)	:arrow_up:
ubuntu-24.04-pypy3.10	91.04% <91.66%> (+0.48%)	:arrow_up:
windows-2022-3.10	91.03% <91.66%> (+0.26%)	:arrow_up:
windows-2022-3.11	90.97% <91.66%> (+0.26%)	:arrow_up:
windows-2022-3.12	91.18% <91.66%> (+0.26%)	:arrow_up:
windows-2022-3.13	91.18% <91.66%> (+0.26%)	:arrow_up:
windows-2022-3.8	91.01% <91.66%> (+0.26%)	:arrow_up:
windows-2022-3.9	90.96% <91.66%> (+0.26%)	:arrow_up:
windows-2022-pypy3.10	91.03% <91.66%> (+0.26%)	:arrow_up:

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