Ensure no two packages have the same git URL

Issue can be found here:
https://atmosphere.meteor.com/package/Amazon%20S3%20File%20Uploader
is the same as
https://atmosphere.meteor.com/package/s3

We need to combat duplicate uploads by identifying packages by their git url, not by their name. To avoid breaking any current functionality, this commit attempts to fix this issue (with a temporary solution) by checking the uniqueness of the git url.