Define clang modules in in_app_purchase plugin to:

1. Allow it to be imported into Swift apps as libraries without use_frameworks!.
2. Allow it to be module imported @import into Objective-C apps.

See further details in #41007

Steps to Reproduce

Import into Swift app as a library

1. flutter create test_create_app
2. Add in_app_purchase to the pubspec.

dependencies:
  flutter:
    sdk: flutter
  in_app_purchase: any

3. In generated ios/Podfile remove use_frameworks!
4. flutter build ios
5. In AppDelegate.swift add import in_app_purchase at the top.
6. Build.

Compilation error: no such module 'in_app_purchase'

Module import (@import) into Objective-C app

1. flutter create -i objc test_create_app
2. Add in_app_purchase.

dependencies:
  flutter:
    sdk: flutter
  in_app_purchase: any

3. flutter build ios
4. In AppDelegate.m add @import in_app_purchase; at the top.
5. Build.

Compilation error: no such module 'in_app_purchase'