dh_dwarf2sources: Copy in copyright information from external sources

This will copy the copyright and copyright_report files for any packages whose files are found inside the resulting binaries, making them available at /usr/share/doc/PACKAGE/PACKAGE_external_licenses_ARCH. Although this task strictly isn't related to dwarf2sources itself, keeping it here is far simpler than adding an extra debhelper script.

https://phabricator.apertis.org/T8554

Signed-off-by: Ryan Gonzalez ryan.gonzalez@collabora.com

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In draft status because this is useless until cargo!14 (merged) is merged.