fix(utils): custom linkable keys (#11400)

**What**
Fix linkable generation when there is no dml models and models are provided as virtual to the joiner config and therefore the linkable are inferred

packages/modules/file/integration-tests/__tests__/module.spec.ts

@@ -1,8 +1,8 @@
-import { resolve } from "path"
-import { moduleIntegrationTestRunner } from "@medusajs/test-utils"
 import { IFileModuleService } from "@medusajs/framework/types"
 import { Module, Modules } from "@medusajs/framework/utils"
+import { moduleIntegrationTestRunner } from "@medusajs/test-utils"
 import { FileModuleService } from "@services"
+import { resolve } from "path"
 
 jest.setTimeout(100000)
 
@@ -28,13 +28,23 @@ moduleIntegrationTestRunner<IFileModuleService>({
           service: FileModuleService,
         }).linkable
 
-        expect(Object.keys(linkable)).toEqual([])
+        expect(Object.keys(linkable)).toEqual(["file"])
 
         Object.keys(linkable).forEach((key) => {
           delete linkable[key].toJSON
         })
 
-        expect(linkable).toEqual({})
+        expect(linkable).toEqual({
+          file: {
+            id: {
+              entity: "File",
+              field: "file",
+              linkable: "file_id",
+              primaryKey: "id",
+              serviceName: "file",
+            },
+          },
+        })
       })
 
       it("creates and gets a file", async () => {