Support uncompressed indexes from partial file:/ mirrors

The initial (fix for a) feature is rather special-interest, but in the process I ran across a couple other related things which have more widely applicable results. The most obvious perhaps is that "Hashsum mismatches" are no longer a complete failure as it is nowadays far less common for users to run into them (due to by-hash and such). APT instead falls back to another mirror instead which is hopefully not as broken.