Has anyone done a major renovation, and had someone help them plan the layout and how to best use the space? Is there a name for that kind of thing?

Space planner

Interior designer (not decorators) will also do this

I would recommend a space planner over an interior designer for this. If you're doing a big project, I recommend BOTH a space planner and interior designer that talks to one another

Yikes, does an Interior Designer not take this into account? I really expected that to be part of their job.

They do to an extent, but depending on how much space you have to waste, you will really want to have someone who specializes in space optimization.

Don't ask an interior designer about colors either! That's an interior decorator's job

Oh man. I guess we need three people!

I'm assuming that once I know what I want (via some combination of space planner and interior designer) that the contractor would then handle things like the architect and all the other specialists that go into creating a plan?

No. A general contractor doesn't manage the architect. You would need to manage the relationship between the architect and the general contractor. If you don't want to manage that relationship, you need a project manager. If you don't want to deal with the headache of getting a project manager, being responsible for finding the right contractor etc, and you want to outsource this whole thing from a to z including design, you are looking for a "design-build firm"

ehh I wish I was in the position to go with the most cost option. Looks like I'll be a project manager, I guess. So - I need to find a space planner (unless I decide to just do that myself, seems like the most likely step to cut out), architect, general contractor - any others?

You absolutely don't need a space planner, but if you want one I have one for you. The only two essentials you need is architect and general contractor. You start with the architect then you'll put the project up for bidding to general contractors (unless this is a small project where you want to just pick your favorite contractor)