pluginupdate.py: fix regression with plugin line splitting
pluginupdate.py: fix regression with plugin line splitting

Before 46c68ad both "@" and " as " could be used in the same line like
the following:

    vimwiki/vimwiki@dev as vimwiki-dev

After 46c68ad this gives an error due to the split URI still erroneously
including the " as [name]" at the end, due to splitting from the wrong
variable.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs