randomize acquire order for same type index files

randomize acquire order for same type index files

Without randomizing the order in which we download the index files we
leak needlessly information to the mirrors of which architecture is
native or foreign on this system. More importantly, we leak the order in
which description translations will be used which in most cases will e.g.
have the native tongue first.

Note that the leak effect in practice is limited as apt detects if a file
it wants to download is already available in the latest version from a
previous download and does not query the server in such cases. Combined
with the fact that Translation files are usually updated infrequently
and not all at the same time, so a mirror can never be sure if it got asked
about all files the user wants.