Module name collision on different apps installed in different sites

Description of the issue

When two independent apps declare a module with the same name in their modules.txt file, Frappe's module mapping system causes ImportError during app installation.

The root cause is in frappe/__init__.py:setup_module_map() which builds the local.module_app dictionary by iterating through apps and overwrites any existing module→app mapping when a duplicate module name is found. This means the last app processed wins, regardless of which app actually owns a specific DocType.

This creates a critical problem:

• Database records (tabModule Def) show the first app that installed the module
• In-memory cache (frappe.local.module_app) points to the last app in iteration order
• Result: Frappe tries to load DocTypes from the wrong app, causing ImportError

Context information (for bug reports)

Output of bench version

frappe 15.x
erpnext 15.x

Affected file: frappe/frappe/__init__.py lines 1649-1688 (setup_module_map)

Steps to reproduce the issue

1. Create two independent apps that both declare a module named "Stock" in their modules.txt:

   • App A (e.g., erpnext): Contains module "Stock" with DocType "Repost Item Valuation"
   • App B (e.g., appcustom): Contains module "Stock" but WITHOUT "Repost Item Valuation"

2. Add both apps to sites/apps.txt in this order:

   frappe
   erpnext
   appcustom
   

3. Create a new site and install App A (erpnext):

   bench new-site testsite
   bench --site testsite install-app erpnext
   

Observed result

Installation fails with:

ImportError: Module import failed for Repost Item Valuation, the DocType you're trying to open might be deleted.
Error: No module named 'appcustom.stock.doctype.repost_item_valuation'

What happens internally:

1. During setup_module_map(), the function iterates through apps in order
2. When processing erpnext: sets local.module_app["stock"] = "erpnext"
3. When proces