Overwrite existing inner class info from JSON.

By ignoring any pre-existing inner class info attempts to edit it,
such as by ForgeGradle handing AccessTransformers on inner classes,
gets ignored.

Some inner classes still work, but many anonymous ones, and some named
inner classes such as the GuiContainerCreative ones have pre-existing
inner class info for some reason.

(This seems to be every inner class in the JSON supplied by Forge with
a "start" attribute)