Lintian tag information: copyright-refers-to-nonexistent-license-file (type: warning)

Description (from lintian-explain-tags)

The copyright file refers to a license in /usr/share/common-licenses that
doesn't exist. Usually this is a typo, such as accidentally omitting the -
between the license name and the version number.

Visibility: warning
Show-Always: no
Check: debian/copyright

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