support apt_sources to be a dictionary

key is the filename, and "old" input shall be handled as it was all the time.
For compatibility this will (continue to) overwrite the file of multiple
options that did not specify an output file (they all get the same default).
Yet it will process them all - as it always did - e.g. to add the keys of all
of them.

Any users of the new format won't have these issues, as they will always have
a key.

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