nobody is stopping you from using it, i think the argument being made isn't that "they had it first," but that a lot of materials are going to conflict with the already-established PHP Composer, and that it presents a possible confusion for your (potential) users.

I'd rename it if it were my project. And "go" is pretty bad, but it's clear that if you're looking for specific resources, 'golang' is the appropriate tag/search term.