OPA REST API decision log path incorrect for non-standard package names

[patrick-east]

Similar to #2027 but more specific to the HTTP server in OPA.

Expected Behavior

With a policy like:

package policy["com.foo.acme.ingress"].main

p := 1

When I make a REST API call to /v1/data/policy/com.foo.acme.ingress/main/p I should see a decision log entry with path set to policy/com.foo.acme.ingress/main/p

Actual Behavior

The decision log path swaps all .'s to /'s so the path ends up being incorrect:

{
  "client_addr": "[::1]:50880",
  "level": "info",
  "msg": "Received request.",
  "req_id": 5,
  "req_method": "GET",
  "req_path": "/v1/data/policy/com.foo.acme.ingress/main/p",
  "time": "2020-01-23T11:43:34-08:00"
}
{
  "decision_id": "9b5e3cf5-d1b1-4114-b680-32d6e5b533b8",
  "labels": {
    "id": "db93a45f-47f6-4a7c-8e43-9fd5aaeed43b",
    "version": "0.17.0-dev"
  },
  "level": "info",
  "metrics": {
    "counter_server_query_cache_hit": 0,
    "timer_rego_input_parse_ns": 362,
    "timer_rego_query_compile_ns": 83207,
    "timer_rego_query_eval_ns": 22537,
    "timer_rego_query_parse_ns": 693133,
    "timer_server_handler_ns": 844860
  },
  "msg": "Decision Log",
  "path": "policy/com/foo/acme/ingress/main/p",
  "requested_by": "[::1]:50880",
  "result": 1,
  "time": "2020-01-23T11:43:34-08:00",
  "timestamp": "2020-01-23T19:43:34.947269Z",
  "type": "openpolicyagent.org/decision_logs"
}

Steps to Reproduce the Problem

1. Run opa server with policy as described above, and decision logger enabled:
   opa run -s --log-format json-pretty --set decision_logs.console=true /tmp/policy.rego
2. Make a request with the data API path including the non-standard package name:
   curl -s localhost:8181/v1/data/policy/com.foo.acme.ingress/main/p
3. Observe the decision log output from the server