fix(topology): handle rebuilding connected components (#9536)

Fixes #9173

This was a bit of a doozy. It turns out that if you do a config reload
that rebuilds two components that are directly connected, Vector will
panic on a broken invariant in `Fanout`.

The problem is basically that:

1. When replacing the upstream component, the new input for the rebuilt
   downstream component has not yet been registered and therefore
   doesn't get readded to the rebuilt `Fanout`.
2. When replacing the downstream component, the input referencing the
   upstream component isn't changing, so we assume we're already
   registered there and need to do an update instead of an insert.
3. When the `Fanout` gets an update (i.e. `ControlMessage::Replace`) for
   a sink that it doesn't have registered, we panic.

This is a very good argument for finding a way to property test our
reloads, as there are likely other edge cases we just haven't run into
yet. It'd also be very helpful to continue pushing towards a more
graph-based config representation internally that could make this logic
far simpler.

Signed-off-by: Luke Steensen <luke.steensen@gmail.com>