Fix project data files detection

[juhoinkinen]

I noticed that after

annif download yso-bonsai-en juhoinkinen/Annif-models-upload-testing

the modification time of omikuji projects as detected by Annif is wrong; it is shown to be the extraction time:

annif show-project yso-bonsai-en 
...
Modification time: 2024-03-13 11:36:22

This is because of the timestamp of the data/projects/yso-bonsai-en/omikuji-model/ directory, which is set to the time when the files it contains get extracted.

Originally posted by @juhoinkinen in #762 (comment)

---

This PR fixes the above problem by making all subdirectories in project data directory to be disregarded as model data files (the directories themselves do not affect the model results).

Also make the files in the subdirectories be regarded as they should be.

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.65%. Comparing base (eba2b0c) to head (47c836c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #778   +/-   ##
=======================================
  Coverage   99.65%   99.65%           
=======================================
  Files          89       89           
  Lines        6404     6405    +1     
=======================================
+ Hits         6382     6383    +1     
  Misses         22       22           

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[osma]

LGTM. I could think of other ways of getting to the same result, but this is certainly good enough.