Fix weak dependencies on renamed packages

The renames of packages ui -> blaze, deps -> tracker,
livedata -> ddp worked fine when a package or an app
dependened on the renamed packages. But if there was
a weak dependency (say `api.use('ui', {weak: true})`)
then:

(1) `Package.ui` would be null (since it's not in
    meteor-platform)
(2) Even if your app added 'ui' explicitly, the 'ui'
    package didn't export any symbols.