Per https://go.dev/ref/mod#go-mod-file-ident, '@' is not a valid module
path character. The old implementation used strings.LastIndex to allow
inputs like github.com/@user/module@v1.0.0 to parse, but such inputs
are not valid module paths per the spec. Reject them explicitly.
Signed-off-by: Mohammed Al Sahaf <msaa1990@gmail.com>
