docs: remove imports from src in medusa-config.js (#8132)

Importing from `src` in `medusa-config.js` works in development, but would cause an error when running `build`/in production.

This removes from the docs all snippets that imports from the `src` directory. However, we should figure out a better way to handle re-using the module's name.

www/apps/book/app/advanced-development/modules/options/page.mdx

@@ -23,13 +23,10 @@ To pass options to a module, add an `options` property to the module’s configu
 For example:
 
 ```js title="medusa-config.js"
-import { HELLO_MODULE } from "./src/modules/hello"
-// ...
-
 module.exports = defineConfig({
   // ...
   modules: {
-    [HELLO_MODULE]: {
+    helloModuleService: {
       resolve: "./modules/hello",
       options: {
         capitalize: true,

www/apps/book/app/basics/modules-and-services/page.mdx

@@ -74,7 +74,7 @@ In `medusa-config.js`, add a `modules` property and pass to it your custom modul
 module.exports = defineConfig({
   // ...
   modules: {
-    "helloModuleService": {
+    helloModuleService: {
       resolve: "./modules/hello",
     },
   },

www/apps/resources/app/recipes/marketplace/examples/vendors/page.mdx

@@ -139,14 +139,10 @@ export default Module(MARKETPLACE_MODULE, {
 Finally, add the module to the list of modules in `medusa-config.js`:
 
 ```ts title="medusa-config.js"
-import { MARKETPLACE_MODULE } from "./src/modules/marketplace"
-
-// ...
-
 module.exports = defineConfig({
   // ...
   modules: {
-    [MARKETPLACE_MODULE]: {
+    marketplaceModuleService: {
       resolve: "./modules/marketplace",
       definition: {
         isQueryable: true,