title: "Ambiguous attribute key" error after updating to terraform version 0.12.26
body: <p>After terraform version update from 0.11 to 0.12.26, I'm seeing error with lookup and list of values inside map.</p>
  <pre><code>variable &quot;foo&quot; {
    type = map
  }
  
  
  foo = {
    x.y = &quot;bar&quot;
  }
  </code></pre>
  <p>I have a map &quot;foo&quot; as variable type (map) and then i have key-value pair in map with x.y = &quot;bar&quot;. In lookup, I'm trying to read value of x.y as,</p>
  <pre><code>lookup(var.foo, x.y)
  </code></pre>
  <p>with this, I'm getting error,</p>
  <pre><code>Error: Ambiguous attribute key
  
    on line 13:
    13:   x.y = &quot;bar&quot;
  
  If this expression is intended to be a reference, wrap it in parentheses. If
  it's instead intended as a literal name containing periods, wrap it in quotes
  to create a string literal.
  </code></pre>
  <p>can someone help?</p>