Support local files as arguments in show command

Now that --with-source is supported in show we can go a little further
and add the "syntactic sugar" of supporting deb-files on the commandline
directly to give users an alternative to remembering dpkg -I for deb
files & as a bonus apt also works on changes files.

Most of the code churn is actually to deal with cases probably not too
common in reality like mixing packages and deb-files on the commandline
and getting the right order for these multiple records.

Closes: 883206

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