Fix build_path to look in domains

[tyarkoni]

Currently, build_path is broken, because the path_patterns in the config files, which used to be stored in the Layout instance, are now tied to individual Domain objects that build_path doesn't check. As a result, Layout.path_patterns is always an empty list. We should add a domains argument to build_path that extracts the specified path_patterns from the passed domains, or uses all available domains if domains=None. See write_contents_to_file for a similar implementation, and bids-standard/pybids#213 for more context.