reorganize -- move elements to root to make importing nicer

as we discussed, move things up to simplify import paths.

This keeps us roughly on par with bower/JavaScript in terms of verbosity:
```html
<link rel="import" href="packages/core_elements/core_input.html">
```
vs
```html
<link rel="import" href="bower_components/core-input/core-input.html">
```

... which one wins on character count will depend on details, but overall it's the same number of tokens and we aren't paying anything extra for grouping them into a single Pub package.