fix(gatsby-source-medusa): Unsupported exports warning and schema (#1463)

**What**

- Moves `sourceUpdatedNodes` and `sourceAllNodes` to a separate file, to prevent warning of unsupported exports. See #1455.
- Adds warnings if `GET /store/products`, `GET /store/regions`, and `GET /store/collections` return empty arrays. This should help new users more easily figure out why their storefronts does not work as expected.
- Adds schema to plugin, so that node types for products, regions and collections are always created. This will prevent errors such as `allMedusaRegions query failed` from happening in the storefront, as the query will be valid as the type exists, even if it returns nothing. This should make the gatsby plugin/starter easier to use. This error is an reoccurring issue in our discord, when new users try to run the Gatsby starter without seeding the storefront beforehand. 

Resolves #1455 

Co-authored-by: Sebastian Rindom <7554214+srindom@users.noreply.github.com>

packages/gatsby-source-medusa/src/source-nodes.ts

@@ -0,0 +1,74 @@
+import { SourceNodesArgs } from "gatsby"
+import { makeSourceFromOperation } from "./make-source-from-operation"
+import { createOperations } from "./operations"
+
+const medusaNodeTypes = [
+  "MedusaRegions",
+  "MedusaProducts",
+  "MedusaOrders",
+  "MedusaCollections",
+]
+
+export async function sourceAllNodes(
+  gatsbyApi: SourceNodesArgs,
+  pluginOptions: MedusaPluginOptions
+): Promise<void> {
+  const {
+    createProductsOperation,
+    createRegionsOperation,
+    createOrdersOperation,
+    createCollectionsOperation,
+  } = createOperations(pluginOptions)
+
+  const operations = [
+    createProductsOperation,
+    createRegionsOperation,
+    createCollectionsOperation,
+  ]
+
+  // if auth token is provided then source orders
+  if (pluginOptions.apiKey) {
+    operations.push(createOrdersOperation)
+  }
+
+  const sourceFromOperation = makeSourceFromOperation(gatsbyApi)
+
+  for (const op of operations) {
+    await sourceFromOperation(op)
+  }
+}
+
+export async function sourceUpdatedNodes(
+  gatsbyApi: SourceNodesArgs,
+  pluginOptions: MedusaPluginOptions,
+  lastBuildTime: string
+): Promise<void> {
+  const {
+    incrementalProductsOperation,
+    incrementalRegionsOperation,
+    incrementalOrdersOperation,
+    incrementalCollectionsOperation,
+  } = createOperations(pluginOptions)
+
+  for (const nodeType of medusaNodeTypes) {
+    gatsbyApi
+      .getNodesByType(nodeType)
+      .forEach((node) => gatsbyApi.actions.touchNode(node))
+  }
+
+  const operations = [
+    incrementalProductsOperation(lastBuildTime),
+    incrementalRegionsOperation(lastBuildTime),
+    incrementalCollectionsOperation(lastBuildTime),
+  ]
+
+  if (pluginOptions.apiKey) {
+    operations.push(incrementalOrdersOperation(lastBuildTime))
+  }
+
+  const sourceFromOperation = makeSourceFromOperation(gatsbyApi)
+
+  for (const op of operations) {
+    await sourceFromOperation(op)
+  }
+}