Bug#907072: lintian: verify AppStream metainfo metadata_license matches debian/copyright

[Daniel Kahn Gillmor <dkg@fifthhorseman.net>]

Package: lintian
Version: 2.5.97
Severity: wishlist

Lintian currently has some checks about appstream-metadata (the
AppStream metainfo xml files shipped with some software).  It also has
some checks about debian/copyright.

The AppStream metainfo files have a member named metadata_license, as
documented here:

   https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-metadata_license

It would be great if lintian could notice that metadata_license
doesn't match the indicated license in debian/copyright.

The gnupg2 source package version 2.2.9-1 has this mismatch because i
was sloppy.  I'll fix it shortly (by relicensing the file to match
what is in d/copyright), but i'd love it if lintian could have helped
me catch my sloppiness earlier :)

           --dkg

-- System Information:
Debian Release: buster/sid
  APT prefers testing-debug
  APT policy: (500, 'testing-debug'), (500, 'testing'), (500, 'oldstable'), (200, 'unstable-debug'), (200, 'unstable'), (1, 'experimental-debug'), (1, 'experimental')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 4.17.0-1-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8), LANGUAGE=en_US.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages lintian depends on:
ii  binutils                          2.31.1-4
ii  bzip2                             1.0.6-9
ii  diffstat                          1.61-1+b1
ii  dpkg                              1.19.0.5+b1
ii  file                              1:5.34-2
ii  gettext                           0.19.8.1-7
ii  intltool-debian                   0.35.0+20060710.4
ii  libapt-pkg-perl                   0.1.34
ii  libarchive-zip-perl               1.62-2
ii  libclass-accessor-perl            0.51-1
ii  libclone-perl                     0.39-1
ii  libdpkg-perl                      1.19.0.5
ii  libemail-valid-perl               1.202-1
ii  libfile-basedir-perl              0.08-1
ii  libipc-run-perl                   20180523.0-1
ii  liblist-moreutils-perl            0.416-1+b3
ii  libparse-debianchangelog-perl     1.2.0-12
ii  libperl5.24 [libdigest-sha-perl]  5.24.1-7
ii  libtext-levenshtein-perl          0.13-1
ii  libtimedate-perl                  2.3000-2
ii  liburi-perl                       1.74-1
ii  libxml-simple-perl                2.25-1
ii  libyaml-libyaml-perl              0.72+repack-1
ii  man-db                            2.8.4-2
ii  patchutils                        0.3.4-2
ii  perl [libdigest-sha-perl]         5.26.2-7
ii  t1utils                           1.41-2
ii  xz-utils                          5.2.2-1.3

Versions of packages lintian recommends:
pn  libperlio-gzip-perl  <none>

Versions of packages lintian suggests:
pn  binutils-multiarch     <none>
ii  dpkg-dev               1.19.0.5
ii  libhtml-parser-perl    3.72-3+b2
ii  libtext-template-perl  1.53-1

-- no debconf information