Identifying the origin of packages

[Tim Beauregard <tim.beauregard@sky.com>]

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I want to remove

deb http://ftp.debian-unofficial.org/debian unstable main contrib
non-free restricted

from my sources.list, however I want to be sure that source isn't
currently providing any packages on my system.  Is there a command that
can list all the packages from each mirror?

I was thinking along the lines of

dpkg -l | grep unofficial

but -l only gives a summary of information.

Thanks in advance,

Tim
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