generate_firmware_patterns: make sure dep11 metadata files are present

Otherwise we might be failing to generate firmware patterns for some
firmware packages. Being cautious about this lets us submit patches
against dak: https://salsa.debian.org/ftp-team/dak/-/merge_requests/267

tools/generate_firmware_patterns

@@ -94,6 +94,8 @@ write_file("$output_dir/README.txt",
 # Components-* files:
 my @all_packages;
 foreach my $component (@ARGV) {
+    die "missing metadata file $component"
+        if ! -f $component;
     my @packages = process_components($component);
     push @all_packages, @packages;
 }