Bug#956723: lintian: add check for invalid UTF8 filenames in source

[Ivo De Decker <ivodd@debian.org>]

package: lintian

Hi,

There is a tag file-name-is-not-valid-UTF-8 for files with invalid UTF-8 in
binary packages. Please add a similar tag for the files with invalid UTF-8 in
source packages.

This was inspired by bug #956233

Note that this causes errors in certain tools that process data from source
packages, like sources.debian.org:

https://sources.debian.org/src/supysonic/0.5.0+ds-2/tests/assets/ currently
gives "500 Something went wrong."

Thanks!

Ivo