Bug#885968: lintian: extra-license-file should ignore sphinx _sources/license.txt

[Stuart Prescott <stuart@debian.org>]

Package: lintian
Version: 2.5.66
Severity: normal

Sphinx always wants to include a copy of the licence of the documentation
within the sources and it always includes the .rst sources of the documentation
in amongst the compiled text. This means that maintainers either end up
ignoring or overriding this tag; there is a cost associated with both.
(Worse, the overridden tag reappears when sphinx changes its file license.txt
to license.rst.txt.)

An example was recently cited on debian-devel@ [1] but with 1391 packages
warning of this and it being overridden 759 times [2], that's a
unacceptably poor hit rate for a lintian tag; lintian is not wrong that there
is a file called license.txt there but it is wrong to assert that the
maintainer could or should do something in this situation.

[1] https://lists.debian.org/debian-devel/2017/12/msg00609.html

[2] https://lintian.debian.org/tags/extra-license-file.html

Lintian could recognise sphinx documentation and not complain about including
the licence within it; not complaining about _sources/license.txt and
_sources/license.rst.txt should achieve that.