Both the set of files that define a mappable BUILD files and the parser used to parse those
files can be customized. See the `pants.engine.exp.parsers` module for example parsers.
:param string build_root: The root of the BUILD files; typically the code repository root
directory.
:param string build_pattern: A regular expression for identifying BUILD files used to resolve
addresses; by default looks for `BUILD*` files.
:param parser_cls: The BUILD file parser cls to use.
:type parser_cls: A :class:`pants.engine.exp.parser.Parser`