fetchHex: remove pname prefix

Projects like mix2nix and my own project, deps_nix, are collecting a
growing number of workarounds for packages that depend on the working
directory of a build to have the same name as the package.

See for example: da2ebc2c63

This change makes the pname of Hex packages equal the package name, so
that such workarounds aren't necessary.

pkgs/development/beam-modules/fetch-hex.nix

@@ -7,7 +7,7 @@
 }:
 
 stdenv.mkDerivation ({
-  pname = "hex-source-${pkg}";
+  pname = pkg;
   inherit version;
   dontBuild = true;
   dontConfigure = true;