Move <nix/fetchurl.nix> into the nix binary

This makes the statically linked nix binary just work, without needing
any additional files.

src/libexpr/fetchurl.nix

@@ -0,0 +1,41 @@
+{ system ? "" # obsolete
+, url
+, hash ? "" # an SRI hash
+
+# Legacy hash specification
+, md5 ? "", sha1 ? "", sha256 ? "", sha512 ? ""
+, outputHash ?
+    if hash != "" then hash else if sha512 != "" then sha512 else if sha1 != "" then sha1 else if md5 != "" then md5 else sha256
+, outputHashAlgo ?
+    if hash != "" then "" else if sha512 != "" then "sha512" else if sha1 != "" then "sha1" else if md5 != "" then "md5" else "sha256"
+
+, executable ? false
+, unpack ? false
+, name ? baseNameOf (toString url)
+}:
+
+derivation {
+  builder = "builtin:fetchurl";
+
+  # New-style output content requirements.
+  inherit outputHashAlgo outputHash;
+  outputHashMode = if unpack || executable then "recursive" else "flat";
+
+  inherit name url executable unpack;
+
+  system = "builtin";
+
+  # No need to double the amount of network traffic
+  preferLocalBuild = true;
+
+  impureEnvVars = [
+    # We borrow these environment variables from the caller to allow
+    # easy proxy configuration.  This is impure, but a fixed-output
+    # derivation like fetchurl is allowed to do so since its result is
+    # by definition pure.
+    "http_proxy" "https_proxy" "ftp_proxy" "all_proxy" "no_proxy"
+  ];
+
+  # To make "nix-prefetch-url" work.
+  urls = [ url ];
+}