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/tools/text/sift/deps.json

@@ -1,10 +1,29 @@
 [
-  {
-    "include": "../../libs.json",
-    "packages": [
-        "github.com/svent/go-flags",
-        "github.com/svent/go-nbreader",
-        "golang.org/x/crypto"
-    ]
-  }
+    {
+        "goPackagePath": "golang.org/x/crypto",
+        "fetch": {
+            "type": "git",
+            "url": "https://go.googlesource.com/crypto",
+            "rev": "575fdbe86e5dd89229707ebec0575ce7d088a4a6",
+            "sha256": "1kgv1mkw9y404pk3lcwbs0vgl133mwyp294i18jg9hp10s5d56xa"
+        }
+    },
+    {
+        "goPackagePath": "github.com/svent/go-flags",
+        "fetch": {
+            "type": "git",
+            "url": "https://github.com/svent/go-flags",
+            "rev": "4bcbad344f0318adaf7aabc16929701459009aa3",
+            "sha256": "1gb416fgxl9gq4q6wsv3i2grq1mzbi7lvfvmfdqbxqbv9vizzh34"
+        }
+    },
+    {
+        "goPackagePath": "github.com/svent/go-nbreader",
+        "fetch": {
+            "type": "git",
+            "url": "https://github.com/svent/go-nbreader",
+            "rev": "7cef48da76dca6a496faa7fe63e39ed665cbd219",
+            "sha256": "0hw11jj5r3f6qwydg41nc3c6aadlbkhc1qpxra2609lis0qa9h4r"
+        }
+    }
 ]