chore(dev): Wrap `vector-core` in `vector-lib` (#18934)

* chore(dev): Wrap `vector-core` in `vector-lib`

This is where things get a little more interesting. Both `vector-common` and
`vector-core` export a `config` module, so we need to create a virtual module
pulling in the relevant symbols. I have made those imports explicit so as to
eliminate the ambiguity of which symbol comes from which package.

* Feature flag fixes