Develop how to fetch packages

[gfngfn]

registry.yaml:

 packages:
 ...
 - name: "std-ja-book"
   implementations:
   - version: "0.0.1"
+    source:
+      type: "tar_gzip"
+      url: "https://gfngfn.github.io/temp/std-ja-book.0.0.1.tar.gz"
     dependencies:
     - name: "stdlib"
       requirements: [ "0.0.1" ]
     - name: "math"
       requirements: [ "0.0.1" ]
     - name: "annot"
       requirements: [ "0.0.1" ]
     - name: "code"
       requirements: [ "0.0.1" ]
     - name: "footnote-scheme"
       requirements: [ "0.0.1" ]
     - name: "font-junicode"
       requirements: [ "0.0.1" ]
     - name: "font-latin-modern"
       requirements: [ "0.0.1" ]
     - name: "font-ipa-ex"
       requirements: [ "0.0.1" ]
     - name: "font-latin-modern-math"
       requirements: [ "0.0.1" ]
 ...