warn when an element is not imported directly

[43081j]

Currently it seems that if you have two elements using the same element, linting will pass if only one imports the element.

So for example:

• a
  • paper-button
• b
  • paper-button

But here, if a uses and does not import paper-button, linting will still pass fine because b has imported it into the tree still.

We should really warn if a doesn't import it directly (its self), because it means if we use this element without importing b one day, it will be wrong.

All used elements should be defined as imports in each module, should they not?