Correctly handle alias/hardlink in content duplication check.

[mgautierfr]

Following openzim/libzim#824 we will need to extend check tools to correctly handle duplication.

• Duplication at entry level (two entries with same content) is valid
• Duplication at blob level (two blobs with the same content) is not valid.

It implies we loop on the blobs instead of the entries to check for duplicated level.
And if we do so, we may want to do other checks at this moment instead of looping on articles after.