Use Python JSON to format packages.json file (#5429)

The packages JSON file which includes the boards, tools, etc. and needs to
have consistent formatting to be reproducible.  The current boards.txt.py
uses a REGEX to string-replace a bit of it, but that bit has a different
indent than the rest of the file.

Use Python's JSON writer to format the whole file repeatably.

package/package_esp8266com_index.template.json

@@ -292,4 +292,4 @@
          "name": "esp8266"
       }
    ]
-}
+}