Bug#664139: lintian should warn about mis-named file in /etc/apt/sources.list.d/

[Adam Heath <doogie@brainfood.com>]

package: lintian
version: 2.5.5~bpo60+1
severity: minor

Reading package lists... Done
N: Ignoring file 'my-common-apt' in directory
'/etc/apt/sources.list.d/' as it has no filename extension

I generally name these files $foo.list, but forgot to in this one
situation.  Lintian should warn in that case.