sentencepiece: split into multiple outputs, optional gperftools

I am using sentencepiece in a downstream application where I want to
minimize the resulting closures. This commit makes changes to make
sentencepiece a leaner dependency:

- Split the outputs, so that the binaries/headers do not end up in the
  transitive closure in a library dependency.

- Add the `withGPerfTools` option, which is enabled by default, to
  make it possible to disable the gperftools dependency. According to
  the sentencepiece README, this dependency gives a 10-40% performance
  improvement. But in many cases this is overshadowed by the neural
  networks that use piece identifiers as input anyway.

pkgs/development/python-modules/sentencepiece/default.nix

@@ -6,10 +6,13 @@
 
 buildPythonPackage rec {
   pname = "sentencepiece";
-  inherit (sentencepiece) version src meta;
+  inherit (sentencepiece) version src;
 
   nativeBuildInputs = [ pkgconfig ];
-  buildInputs = [ sentencepiece ];
+  buildInputs = [ sentencepiece.dev ];
 
   sourceRoot = "source/python";
+
+  # sentencepiece installs 'bin' output.
+  meta = builtins.removeAttrs sentencepiece.meta [ "outputsToInstall" ];
 }