implement 'apt-get files' to access index targets

Downloading additional files is only half the job. We still need a way
to allow external tools to know where the files are they requested for
download given that we don't want them to choose their own location.

'apt-get files' is our answer to this showing by default in a deb822
format information about each IndexTarget with the potential to filter
the records based on lines and an option to change the output format.

The command serves also as an example on how to get to this information
via libapt.

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