rustcSrc: remove unrelated files

This package is used to provide semantic completion in tools like racer
and is equivalent to rust-src in rustup. This commit gets rid of a
number of files not required for completion.

pkgs/development/compilers/rust/rust-src.nix

@@ -0,0 +1,11 @@
+{ stdenv, rustc }:
+
+stdenv.mkDerivation {
+  name = "rust-src";
+  src = rustc.src;
+  phases = [ "unpackPhase" "installPhase" ];
+  installPhase = ''
+    mv src $out
+    rm -rf $out/{ci,doc,driver,etc,grammar,llvm,rt,rtstartup,rustllvm,test,tools,vendor}
+  '';
+}