Pass single result tree to modes in wrapper

When matching a `<xslt:include>` inside of a `书社:metadata` transform,
it’s useful to be able to easily match elements in the result. Right
now, that requires a reference to the little‐documented variable
`$书社:result`, because `<xslt:include>` elements don’t belong to the
same document as result tree elements.

This commit creates a new tree which simply copies over all the result
nodes and all of the `<xslt:include>` elements, and then passes that to
the modal templates. Consequently, matches like `/html:div` or
`/xslt:include` should work regardless of what the current context node
is.