configs/configupgrade: Retain any .tf.json files unchanged

We don't change JSON files at all and instead just emit a warning about
them since JSON files are usually generated rather than hand-written and
so any updates need to happen in the generator program rather than in its
output.

However, we do still need to copy them verbatim into the output map so
that we can keep track of them through any subsequent steps.

configs/configupgrade/upgrade.go

@@ -93,6 +93,7 @@ func (u *Upgrader) Upgrade(input ModuleSources) (ModuleSources, tfdiags.Diagnost
 					name,
 				),
 			})
+			ret[name] = src // unchanged
 			continue
 		}