buildGoPackage: remove Go deps file libs.json

After #16017 there were a lot
of comments saying that `nix` would be better than `JSON`
for Go packages dependency sets.
As said in https://github.com/NixOS/nixpkgs/pull/16017#issuecomment-229624046

    > Because of the content-addressable store, if two programs have the
    > same dependency it will already result in the same derivation in
    > the
    > store. Git also has compression in the pack files so it won't make
    > much difference to duplicate the dependencies on disk. And finally
    > most users will just use the binary builds so it won't make any
    > differences to them.

This PR removes `libs.json` file and puts all package dependencies in
theirs `deps.json`.

pkgs/development/tools/gotools/deps.json

@@ -1,8 +1,11 @@
 [
-  {
-    "include": "../../libs.json",
-    "packages": [
-      "golang.org/x/net"
-    ]
-  }
+    {
+        "goPackagePath": "golang.org/x/net",
+        "fetch": {
+            "type": "git",
+            "url": "https://go.googlesource.com/net",
+            "rev": "62ac18b461605b4be188bbc7300e9aa2bc836cd4",
+            "sha256": "0lwwvbbwbf3yshxkfhn6z20gd45dkvnmw2ms36diiy34krgy402p"
+        }
+    }
 ]