Allow external dependencies in registries

[gfngfn]

(See https://github.com/gfngfn/SATySFi/pull/422)

You will be able to use external registries in your package release configs:

 saphe: "^0.0.1"
 satysfi: "^0.1.0"
 name: "your-cool-pkg"
 version: "0.0.1"
 source:
   tar_gzip:
     url: "https://foo.com/your-cool-pkg.0.0.1.tar.gz"
     checksum: "52fb5bf621027c218c2522d4ccb1e375"
+registries:
+- name: "an-external-registry"
+  git:
+    url: "https://bar.com/an-external-registry"
+    branch: "master"
 dependencies:
 - used_as: "Stdlib"
   name: "stdlib"
   requirement: "^0.0.1"
 - used_as: "YourLibrary"
+  registry: "an-external-registry"
   name: "your-library"
   requirement: "^0.0.1"