RezPlugin package_repository breaks when @ in repo path

>
> The issues is that when a package repo path looks like 'foo@/bah', it is interpreted as a repo where package payloads are stored on filepath /bah, and the plugin type is 'foo'.
>
> I think it would suffice to only interpret the prefix as a plugin type if it matches a restrictive regex (alphanumeric/underscore).

https://groups.google.com/forum/#!topic/rez-config/Z7NdidsJNUY

Start by locating the RezPlugin package_repository parsing entry point and reproduce the reported case with a repository path like `foo@/bah`. Confirm that the prefix is treated as a plugin type only when it matches the proposed restrictive alphanumeric/underscore pattern, and verify that the path remains intact otherwise.

Written by the indexing model from the issue text.