Add MVTec backward compatibility

[samet-akcay]

📝 Description

• After renaming MVTec to MVTecAD, the relative import path to import MVTec is broken. This PR reverts that functionality back for compatibility.

# Before 
from anomalib.data import MVTec

# After PR#2557
from anomalib.data import MVTec # ❌ Broken
from anomalib.data import MVTecAD

# Now
from anomalib.data import MVTec    # ✅ Deprecated, but works
from anomalib.data import MVTecAD

✨ Changes

Select what type of change your PR is:

• 🐞 Bug fix (non-breaking change which fixes an issue)
• 🔨 Refactor (non-breaking change which refactors the code base)
• 🚀 New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📚 Documentation update
• 🔒 Security update

✅ Checklist

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• 📋 I have summarized my changes in the CHANGELOG and followed the guidelines for my type of change (skip for minor changes, documentation updates, and test enhancements).
• 📚 I have made the necessary updates to the documentation (if applicable).
• 🧪 I have written tests that support my changes and prove that my fix is effective or my feature works (if applicable).

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