getSourceExpr(): Handle channels
getSourceExpr(): Handle channels

Fixes #1892.
Fixes #1865.
Fixes #3119.

dtzWill/nix,NixOS/nix,rycee/nix,rycee/nix,pikajude/nix,pikajude/nix,ehmry/nix,NixOS/nix,ehmry/nix,NixOS/nix,NixOS/nix,dtzWill/nix,NixOS/nix,rycee/nix,ehmry/nix,rycee/nix,dtzWill/nix,ehmry/nix,rycee/nix,ehmry/nix,ehmry/nix,dtzWill/nix,pikajude/nix,dtzWill/nix