dh_setup_copyright: Improve perf of handling non-existent source files

find_package_providing_path would try to look up the source package for
every incoming file, even if the file doesn't actually exist on disk.
This is largely useless, because dpkg-query only finds matches for
*installed* packages anyway, so just skip calling it at all for
non-existent files.

Signed-off-by: Ryan Gonzalez <ryan.gonzalez@collabora.com>

dh_setup_copyright

@@ -94,7 +94,11 @@ sub find_package_providing_path {
 		return $package eq PATH_HAS_NO_PACKAGE ? undef : $package;
 	}
 
-	($package) = `dpkg-query -S "$path" 2>/dev/null` =~ /^([^\s:]+):(?:[^\s:]+:)? /;
+	# Skip non-existent files, since dpkg-query only works on installed packages
+	# anyway.
+	if (-e $path) {
+		($package) = `dpkg-query -S "$path" 2>/dev/null` =~ /^([^\s:]+):(?:[^\s:]+:)? /;
+	}
 	if (!defined $package) {
 		$external_files_to_packages_cache{$path} = PATH_HAS_NO_PACKAGE;
 		return undef;