use "source" for source names

this removes a difference to how Nix currently fetches flake references.

using a constant is preferrable because renaming the source won't force downstream rebuilds.

there probably is no correct solution here, as any choice for the
symbolic name is essentially arbitrary, and arguably symbolic names were
an unfortunate design decision anyway.

nix/sources.nix

@@ -7,22 +7,16 @@ let
   #
 
   fetch_file = pkgs: name: spec:
-    let
-      name' = sanitizeName name + "-src";
-    in
     if spec.builtin or true then
-      builtins_fetchurl { inherit (spec) url sha256; name = name'; }
+      builtins_fetchurl { inherit (spec) url sha256; name = "source"; }
     else
-      pkgs.fetchurl { inherit (spec) url sha256; name = name'; };
+      pkgs.fetchurl { inherit (spec) url sha256; name = "source"; };
 
   fetch_tarball = pkgs: name: spec:
-    let
-      name' = sanitizeName name + "-src";
-    in
     if spec.builtin or true then
-      builtins_fetchTarball { name = name'; inherit (spec) url sha256; }
+      builtins_fetchTarball { name = "source"; inherit (spec) url sha256; }
     else
-      pkgs.fetchzip { name = name'; inherit (spec) url sha256; };
+      pkgs.fetchzip { name = "source"; inherit (spec) url sha256; };
 
   fetch_git = name: spec:
     let
@@ -69,16 +63,6 @@ let
   # Various helpers
   #
 
-  # https://github.com/NixOS/nixpkgs/pull/83241/files#diff-c6f540a4f3bfa4b0e8b6bafd4cd54e8bR695
-  sanitizeName = name:
-    (
-      concatMapStrings (s: if builtins.isList s then "-" else s)
-        (
-          builtins.split "[^[:alnum:]+._?=-]+"
-            ((x: builtins.elemAt (builtins.match "\\.*(.*)" x) 0) name)
-        )
-    );
-
   # The set of packages used when specs are fetched using non-builtins.
   mkPkgs = sources: system:
     let