Softlink publishing should support same reference ID from different providers

Description

Steps to reproduce:

1. Create a Alloy site.
2. Configure a custom content provider (for example, an XML content provider).
3. Create a content item on the default content provider with ID XXX and publish it.
4. Create a content item on the custom content provider with the same ID XXX and publish it.
5. Create a content item that includes an XML html string property.
6. Add content link (XXX) and custom content link into XML html string property.
7. Publish the content item from step 6.

Expected result:

Publish should be done without an exceptions.

Actual result:

Publish throws exception (item already exist).