Add support for include '(@)' in project.conf and .bst files

buildstream/_exceptions.py

buildstream/_frontend/app.py

buildstream/_frontend/widget.py

buildstream/_includes.py

buildstream/_loader/loader.py

buildstream/_loader/metaelement.py

buildstream/_loader/metasource.py

buildstream/_options/optionpool.py

buildstream/_project.py

buildstream/_stream.py

buildstream/_yaml.py

buildstream/element.py

buildstream/source.py

doc/source/format_intro.rst

tests/artifactcache/config.py

tests/format/include.py

tests/format/include/conditional/element.bst

tests/format/include/conditional/extra_conf.yml

tests/format/include/conditional/project.conf

tests/format/include/file/element.bst