gopls: don't build integration tests or documentation generator

Not only are these not used, but they also have super generic names,
which is confusing when they appear on the path, and possibly leads
to conflicts with other binaries.

pkgs/development/tools/gopls/default.nix

@@ -15,6 +15,9 @@ buildGoModule rec {
 
   doCheck = false;
 
+  # Only build gopls, and not the integration tests or documentation generator.
+  subPackages = [ "." ];
+
   meta = with stdenv.lib; {
     description = "Official language server for the Go language";
     homepage = "https://github.com/golang/tools/tree/master/gopls";