Basic implementation of <import>

This CL adds basic support for the <import> elements. We're using the same imports
machinery as <link rel="import">, which simplifies this patch substantially.

Currently we support both <link rel="import"> and <import>. Once this CL lands, I'll
update all the existing modules and then we can drop support for
<link rel="import">.

R=ojan@chromium.org

Review URL: https://codereview.chromium.org/681983005