datatransfer.{cc,hh} -> filetransfer.{cc,hh}

src/libexpr/common-eval-args.cc

src/libexpr/eval.cc

src/libexpr/parser.y

src/libstore/build.cc

src/libstore/builtins/fetchurl.cc

src/libstore/filetransfer.cc

src/libstore/filetransfer.hh

src/libstore/http-binary-cache-store.cc

src/libstore/s3-binary-cache-store.cc

src/nix-channel/nix-channel.cc

src/nix-prefetch-url/nix-prefetch-url.cc

src/nix/main.cc

src/nix/upgrade-nix.cc