Handle overlap between package paths and rule names

[tsandall]

If a package path overlaps with a rule name (or vice versa), OPA should handle it better.

It seems reasonable to treat this kind of overlap as a conflict and reject at compile-time.

Simple example:

test1.rego...

package x

p = "hello"

test2.rego...

package x.p

q = "goodbye"

queries...

[venv/go](master)[*]torin:~/go/src/github.com/open-policy-agent/opa$ ./opa_darwin_amd64 run ~/test*
OPA 0.1.1-dev (commit b033f78-dirty, built at 2016-11-03T21:24:35Z)

Run 'help' to see a list of commands.

> data.x
{
  "p": "hello"
}
> data.x.p
"hello"
> data.x.p.q
"goodbye"