Deduplicate transferred / moved repos

[kiskoza]

Description

It's pretty confusing on the landing page that it's full of duplicated repositories, e.g. in the attached screenshot

The code already had a Todo in place, and some of the issues / PRs mentioned code snippets used during repository cleanups, I put them into this PR.

Related Issue

#1704 (comment)
#824 (comment)
#1791 (and a few others...)

Motivation and Context

It helps people find the right repository, for example mozilla/rust has 244 subscribers and rust-lang/rust has 308, though these two repos are the same.

How Has This Been Tested?

I updated the existing test suite.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.