debListParser: Micro-optimize AvailableDescriptionLanguages()

Generating a string for each version we see is somewhat inefficient.
The problem here is that the Description tag names are longer than
15 byte, and thus require an allocation on the heap, which we should
avoid.

It seems reasonable that 20 characters works for all languages codes
used for archive descriptions, but if not, there's a warning, so
we'll catch that.

This should improve performance by about 2%.

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