Clarified yarn link Warning

[irrationalRock]

Summary
This PR fixes #5991.

The message for linkCollision is now:

There's already a package called $0 registered. This command has had no effect. If this command was run in another folder with the same name,  the other folder is still linked. Please run yarn unlink in the other folder if you want to register this folder.

Motivation
As mentioned in the issue, the current warning for yarn link is vague and should be more specific.

Test plan
When running yarn link, it produces this warning now:

warning There's already a package called "rimraf" registered. This command has had no effect. If this command was run in another folder with the same name, the other folder is still linked. Please run yarn unlink in the other folder if you want to register this folder.

[arcanis]

Thanks!