Update babel-import-util and use it to manage babel bindings more precisely

[ef4]

This switches most of our usage to the new automatic-reference APIs in babel-import-util.

The only remaining place where we're responsible for managing the references ourselves is ones introduced by jsutils.bindImport. But we're already responsible for managing references to everything in the scope bag anyway, so that is appropriate.

This refactors the scope-bag-reference management so it happens more precisely, directly against the scope bag (in 'hbs' target mode) or the locals array (in 'wire' target mode). This should result in a slightly more efficient plugin, since we're recrawling a small set of the AST to establish references.