Change common semantic segmentation dataset detection rule

[itrushkin]

Since Datumaro 1.8, the following folder structure:

# Common semantic segmentation dataset example from OTX tests
.
└── supervised
    ├── test
    │   ├── dataset_meta.json
    │   ├── images
    │   │   ├── 0001.png
    │   │   └── 0002.png
    │   └── masks
    │       ├── 0001.png
    │       └── 0002.png
    ├── train
    │   ├── dataset_meta.json
    │   ├── images
    │   │   ├── 0001.png
    │   │   ├── 0002.png
    │   │   └── 0003.png
    │   └── masks
    │       ├── 0001.png
    │       ├── 0002.png
    │       └── 0003.png
    └── val
        ├── dataset_meta.json
        ├── images
        │   ├── 0001.png
        │   └── 0002.png
        └── masks
            ├── 0001.png
            └── 0002.png

is detected as both common_semantic_segmentation and common_semantic_segmentation_with_subset_dirs. Since they are detected at different levels, the current implementation could not set the higher priority to the second one.

This PR updates detection rules for the common_semantic_segmentation format, searching for requirements in the root directory only.

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Project coverage is 81.04%. Comparing base (be7f526) to head (10cd400).
Report is 18 commits behind head on develop.

Files	Patch %	Lines
...ugins/data_formats/common_semantic_segmentation.py	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1572      +/-   ##
===========================================
- Coverage    81.04%   81.04%   -0.01%     
===========================================
  Files          278      278              
  Lines        32489    32490       +1     
  Branches      6600     6600              
===========================================
  Hits         26330    26330              
  Misses        4706     4706              
- Partials      1453     1454       +1     

Flag	Coverage Δ
ubuntu-20.04_Python-3.10	81.02% <85.71%> (-0.01%)	⬇️
windows-2022_Python-3.10	81.01% <85.71%> (-0.01%)	⬇️

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[jihyeonyi]

LGTM.
However, I think related test should be modified, but I guess there's no test affected by this modification. Could you add a related test later?

[jihyeonyi]

LGTM.