[modular] allow multiple modular files in the same model folder

[Cyrilvallez]

What does this PR do?

As per the title. It fixes the issue mentioned in #36261. There are 2 small issues:

• we were relying on the folder name to get the model name -> of course this fails if several modular files are in the same folder, so we now parse the full filename instead
• Small prefix issue, in case the two models (the new one, and the one imported) have a common name pattern (i.e. in this case, rt_detr_resnet and d_fine_resnet both end with resnet, the prefix matching would lead to inconsistencies

[HuggingFaceDocBuilderDev]

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