Avoid launch dependency on lark

[ivanpauno]

Feature request

Feature description

#226 added a dependency on lark to launch:

launch/launch/package.xml

Lines 10 to 13 in 15af530

	<depend>osrf_pycommon</depend>
	<exec_depend>ament_index_python</exec_depend>
	<exec_depend>python3-lark-parser</exec_depend>

.
It can be easily avoided moving the substitution parser to an independent package.

Implementation considerations

launch_xml, launch_yaml can then import this package and override the Parser parse_substitution method.

[ivanpauno]

@wjwwood @hidmic FYI