Make debhelper config files without a package prefix an error in compat 15

Starting in compat 14, it will now trigger a warning to have debian/install instead of debian/package.install. In compat 15, this is promoted to an error. The main exceptions are files like debian/changelog, debian/NEWS, and debian/copyright, which are applied to all packages and can still be used without a prefix.

Closes: #1014537 Closes: #871795 Gbp-Dch: Full Signed-off-by: Niels Thykier niels@thykier.net