Partial product may be produced even if full (`txxxxxx`) ones exist

In some scenarios:

• a txxxxxx concatenation of two orthorectified products already exist for a requested S2 tile
• only one of the two S1 products, that cover that requested S2 tile, exist on disk (it could have been downloaded for an adjacent S2 tile)

Then

• S1Tiling will avoid downloading the missing S1 product (as the txxxxxx file exists),
• but it will produce a thhmmss product made from only one (the existing one) S1 product.

The reason lies in does_product_exist heuristics that checks whether a file of the expected filename already exists. In partially concatenated file cases, we need to check whether the related txxxxxx file exists as well.