Please make com.gradle.plugin-publish open source

### Expected Behavior
The plugin publishing plugin should be open sourced since it is used by many gradle plugins to publish to a repository.

### Current Behavior
The plugin currently is in an internal repository.

### Context
I am trying to come up with a solution to package java programs that use gradle as their build system in Gentoo Linux. As most users build everything from source, using the internal plugin cannot be avoided which might become an issue for users who want their whole system to be built from source and not have any proprietary/internal code.