Avoid the 'ResourceLoaderGetLessVars' hook, which registers a
global LESS variable exposed to all modules (inc. core/extensions).
Instead, use a FileModule subclass to provide the variable only
to the modules that need it.
Using this approach, also has the benefit of being tracked by
the version hasher, so that changes to the configuration actually
result in timely invalidation of module caches.
Bug: T171367
Change-Id: I7c7d13c83004edfc852a9b7dbfc4169617ab9f64
