fix: Change RepoModule into a union with oneOf

.gitignore

@@ -1,4 +1,4 @@
-tsp-output/*
-node_modules/*
-dist/*
-src-tsp/modules/*
+tsp-output/
+node_modules/
+dist/
+src-tsp/modules/

fetchModuleSchemas.js

@@ -41,5 +41,12 @@ for (const module of modules) {
 
 fs.writeFileSync(`${modulesDir}/index.tsp`, 
 moduleImports.map(m => `import "./${m}";`).join("\n") + `
-alias RepoModule = ${moduleModels.map(m => `${m}Module`).join(" | ")};`
+import "@typespec/json-schema";
+
+using TypeSpec.JsonSchema;
+
+@oneOf
+union RepoModule {
+${moduleModels.map(m => `  ${m}Module`).join(",\n")}
+}`
 )