Recent changes in webpack modified the content of the JSON stats object
when compiling assets in production mode. Notably a module chunk is now
identified by an integer instead of its name.
So add an extra mapping between a chunk id and its name in order to fix
the generated Web Labels table.
This plugin definitely needs to have some tests.
@vlorentz started this work in D1215 but I will create a task to not forget
about it.