Multiple package directives allowed by parser

[tsandall]

The parser currently allows modules to include multiple package directives. Only the first package directive will be processed, others are ignored. This is confusing and should not be allowed. Modules must contain exactly one package directive.

For example:

test.rego:

package xyz
package abc

opa run -s

curl -X PUT localhost:8181/v1/policies/test --data-binary @test.rego
{
  "ID": "test",
  "Module": {
    "Package": {
      "Path": [
        {
          "Type": "var",
          "Value": "data"
        },
        {
          "Type": "string",
          "Value": "xyz"
        }
      ]
    },
    "Imports": null,
    "Rules": []
  }
}

[tsandall]

This should only require a relatively small fix in the parser:

https://github.com/open-policy-agent/opa/blob/master/ast/parser_ext.go#L329