Chore/rm main entity concept (#7709)

**What**
Update the `MedusaService` class, factory and types to remove the concept of main modules. The idea being that all method will be explicitly named and suffixes to represent the object you are trying to manipulate.
This pr also includes various fixes in different modules

Co-authored-by: Stevche Radevski <4820812+sradevski@users.noreply.github.com>
Co-authored-by: Oli Juhl <59018053+olivermrbl@users.noreply.github.com>

integration-tests/modules/__tests__/payment/payment-session.workflows.spec.ts

@@ -31,7 +31,7 @@ medusaIntegrationTestRunner({
         let paymentCollection
 
         beforeEach(async () => {
-          region = await regionModule.create({
+          region = await regionModule.createRegions({
             currency_code: "usd",
             name: "US",
           })
@@ -130,7 +130,7 @@ medusaIntegrationTestRunner({
               },
             })
 
-            const region = await regionModule.create({
+            const region = await regionModule.createRegions({
               currency_code: "usd",
               name: "US",
             })