AssetGraphComputation inside AssetsDefinition (#22554)

## Summary & Motivation

Sprouting off of Nick's comment
[here](#22165 (comment)),
this moves all the computation-related properties of `AssetsDefinition`
into their own class.

While in this PR it's restricted to the internals of `AssetsDefinition`,
I think that this object could eventually take on a wider role. It
basically corresponds to the nodes on the right in this diagram I've
been bandying about:

<img width="860" alt="image"
src="https://github.com/dagster-io/dagster/assets/654855/69161dc4-dc23-4a35-a798-73e5548eb216">


## How I Tested These Changes