fix: path in Debian md5sums file (#1052)

When packaging a Debian file, I noticed:

```bash
$ cat /var/lib/dpkg/info/<my-pkg>.md5sums
d53cafdbafeaa62df89f21a80ccb7c1e  ./etc/<my-pkg>/<some-file>.yml
```

From the [Debian md5sums manpages](https://manpages.debian.org/testing/dpkg-dev/deb-md5sums.5.en.html):

> This file contains a list of MD5 digests (as 32 case-insensitive
> hexadecimal characters) followed by two spaces (U+0020 SPACE) and
> the absolute pathname of a plain file, one per line.
>
> Trailing slashes (U+002F /) in the pathname will be trimmed.
> Neither trailing whitespace nor empty or whitespace-only
> lines are accepted.

So, a proper file should contain lines like (note no `./`):

```
d53cafdbafeaa62df89f21a80ccb7c1e  etc/<my-pkg>/<some-file>.yml
```

I do not know Go but I was able to come up with this fix which
works at least for my use case.

deb/deb.go

@@ -505,7 +505,7 @@ func copyToTarAndDigest(file *files.Content, tw *tar.Writer, md5w io.Writer) (in
 	if _, err := io.Copy(tw, io.TeeReader(tarFile, digest)); err != nil {
 		return 0, fmt.Errorf("%s: failed to copy: %w", file.Source, err)
 	}
-	if _, err := fmt.Fprintf(md5w, "%x  %s\n", digest.Sum(nil), header.Name); err != nil {
+	if _, err := fmt.Fprintf(md5w, "%x  %s\n", digest.Sum(nil), files.AsRelativePath(header.Name)); err != nil {
 		return 0, fmt.Errorf("%s: failed to write md5: %w", file.Source, err)
 	}
 	return file.Size(), nil