Add support for structured output from publish goal.

[kaos]

This allows publisher plugins to provide additional meta data about what has been published, along with some common default data from the core publish goal rule.

Example output:

$ ./pants publish --output=publish-result.json testprojects/src/python/docker:: 
14:41:46.65 [INFO] Initializing scheduler...
14:41:46.88 [INFO] Scheduler initialized.
14:41:51.98 [INFO] Built docker image: docker.registry.example.net:8181/test-example:1.2.5
The push refers to repository [docker.registry.example.net:8181/test-example]
7c521f914a2d: Layer already exists 
cd61869a0a0d: Layer already exists 
da8ee6936a47: Layer already exists 
6ae392c466cc: Layer already exists 
7fcd2600f5ad: Layer already exists 
8f56c3340629: Layer already exists 
ba6e5ff31f23: Layer already exists 
9f9f651e9303: Layer already exists 
0b3c02b5d746: Layer already exists 
62a747bf1719: Layer already exists 
1.2.5: digest: sha256:961beb6126320d277ad4236f9f12694ad4e077a16f73b896b3292023007a002c size: 2428

✓ docker.registry.example.net:8181/test-example:1.2.5 published.
$ cat publish-result.json 

[
  {
    "exit_code": 0,
    "names": [
      "docker.registry.example.net:8181/test-example:1.2.5"
    ],
    "published": true,
    "publisher": "docker",
    "registries": [
      "docker.registry.example.net:8181"
    ],
    "status": "published",
    "target": "testprojects/src/python/docker:test-example"
  }
]

[stuhood]

Thanks!

[stuhood]

Since this is only executed when publish.output is set, and since backends can include arbitrary data, I'm a bit worried that this will bitrot. Is it possible to make the encoded data typesafe? If not, should we unconditionally render the JSON, but only write it if there is a target file?

[kaos]

Ah, good point. How about always render it, and log it to the pants log (how's that done?) so you can inspect it for debugging purposes..

[kaos]

Resorted to simply debug log it for now..