alias does not open namespaces

As opposed to `def` and `abbrev`, `alias` does not open the namespaces of the declaration it is in:
```
import Batteries.Tactic.Alias

def Foo.bar : Nat := 0

alias Foo.baz := bar -- unknown constant 'bar'
def Foo.qux := bar -- works
abbrev Foo.quux := bar -- works
```