These attributes are only accessed at a time when performance isn't
critical (e.g. tests), so we can skip caching them and read each
extension.json file again when we do need them.
At the same time, we want to reduce the size of the array that we're
caching due to PHP 7 performance regressions with APC (T187154).
'QUnitModules' is the first attribute to be converted into a lazy-loaded
attribute.
Bug: T220994
Change-Id: I224cdace05dbac00ff947c41daecb74b07c967bb
d6390ba1ee