Links the content found at source_path and represents a Block that represents the content.

def link(source_path):
    """Links the content found at source_path and represents a Block that represents the content."""
    if not os.path.isfile(source_path):
        raise SourceNotFound(source_path)
    with open(source_path, 'r') as f:
        content = f.read()
    block_map = BlockMap()  # The map will be populated with the following function call.
    all_block = convert_lines_to_block(
            content.splitlines(), block_map, LinkStack(source_path), source_path)
    return all_block, block_map.get_variables()