Add structured output to tach report

[pavelzw]

It would be nice if tach report also supported json output.

Something like this

$ tach report --help
...
  --output        Output format, one of `pretty`, `raw`, `json`

tach report --json my_module
{
  "dependencies": [
    {
      "file_path": "my_module/abc.py",
      "line_number": 44,
      "import": "other_module.abc"
    },
    // ...
  ],
  "usages": ...,
  "external-dependencies": ...
}