This file is indexed.

/usr/lib/dpkg/methods/disk/desc.harddisk is in dselect 1.18.25.

This file is owned by root:root, with mode 0o644.

The actual contents of the file can be viewed below.

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Installation from filesystem on a hard disk partition that's not
currently mounted.

The partition should contain the Packages.gz file from each
distribution area being installed (usually stable, contrib and
non-free) as well as the corresponding binary/*/*.deb files.

The easiest way to get this is to make a (partial) copy of the
distribution site's directory hierarchy, if possible.