REPL incorrectly creating rules for some input refs

[tsandall]

The partial doc shorthand improvement (#425) caused the REPL to interpret some refs to input docs as rule definitions when they should not be. For example:

OPA 0.5.6-dev (commit 4a4af6b, built at 2017-08-29T17:50:50Z)

Run 'help' to see a list of commands.

> input.p
> input
[
  "p"
]
> unset input
> input.p = 1
> show
package repl

input["p"] = 1

version = {
	"Version": "0.5.6-dev",
	"BuildCommit": "4a4af6b",
	"BuildTimestamp": "2017-08-29T17:50:50Z",
	"BuildHostname": "testing-gce-a89e468d-2e98-487a-8a82-deea63e462ab",
}