private static List<String> getDefLines(Reader in) {
        if (null == in) {
            throw new IllegalArgumentException("Null reader.");
        }
        List<String> defLines = new ArrayList<>();
        LineNumberReader reader;
        try {
            if (in instanceof LineNumberReader) {
                reader = (LineNumberReader) in;
            } else {
                reader = new LineNumberReader(in);
            }
            String line;
            while ((line = reader.readLine()) != null) {
                line = line.trim();
                if (!line.startsWith("#") && !line.equals("")) {
                    defLines.add(line);
                }
            }
            reader.close();
        } catch (IOException e) {
            throw new RuntimeException("IOException", e);
        }
        return defLines;
    }
Finds the def package or namespace from a reader for a def-file. Returns "" (empty string)
if no package or namespace was found. If both package and namespace are declared in the def
file, the package is returned.

@param in A reader to a def-file
@return namespace of the def-file, or "" (empty string) if no namespace was found
config/src/main/java/com/yahoo/vespa/config/util/ConfigUtils.java