Allow adding custom apps with same name as marketplace app

[balamurali27]

In new site dialog, marketplace app of the same name overrides custom app if they happen to have the same name

ref: https://support.frappe.io/helpdesk/tickets/54171

[BreadGenie]

Headsup: If we enable it, this can be an easy workaround to use paid apps.
Fork and use as custom app.