Match module id to directory name convention

Other module manifests (spine-prototype, vagrant-skeleton, etc.) use
the short directory name as the id, not the long 'sporel-module-X'
form which is reserved for the Gitea slug. The mismatch caused the
launcher script (which exports SPOREL_MODULE=map-editor) to fail
with ENGINE_ERR_MODULE_NOT_FOUND.

manifest.module

@@ -1,5 +1,5 @@
 {
-    "id": "sporel-module-map-editor",
+    "id": "map-editor",
     "version": "0.1.0",
     "kind": "module",
     "api": "^0.1",