Debian sources.list syntax should apply to all files with .list extension

[gszymaszek]

Currently only the files having name that ends with sources.list are highlighted using debian-sources-list.syntax. In Debian it is common to place more sources.list entries in separate files in the /etc/apt/sources.list.d directory (like deb-multimedia.list), but those files usually don’t match the sources.list$ regex.

From sources.list(5):

The filenames may only contain letters (a-z and A-Z), digits (0-9), underscore (_), hyphen (-) and period (.) characters. (…) Files (…) have the extension .list.

I think there are at least two solutions to this issue:

1. match files that end with .list or
2. match files that end with sources.list or that end with .list, but are located in a directory named sources.list.d.