Add a hard-coded exception for sproxy, which is not registered on Hackage for some reason.
Add a hard-coded exception for sproxy, which is not registered on Hackage for some reason.

Cc: da39a3ee5e6b4b0d3255bfef95601890afd80709@shlevy

psibi/cabal2nix,dmalikov/cabal2nix,Fuuzetsu/cabal2nix,bennofs/cabal2nix,jb55/cabal2nix,spencerjanssen/cabal2nix,gridaphobe/cabal2nix