Stop importing/exposing everything that can be used by end-users in Cosmos __init__

[tatiana]

The current approach is very error-prone:
https://github.com/astronomer/astronomer-cosmos/blob/aa26fc35b7caf5e456e1d582ebab4056c5bfb34f/cosmos/__init__.py

People who add new execution modes have to change multiple places, and the tendency is for this module to grow indefinitely. We should stop doing this and clean up what we want to expose from within the root path of Cosmos.

[pankajkoti]

Should we create a label saying breaking-change and attach to such issues?

[tatiana]

I think this is a great suggestion, @pankajkoti , could you do this, please?

[pankajkoti]

Thanks @tatiana for the agreement. I have created a label and attached to the ones that I felt are breaking changes(most of them already had Cosmos 2.0.0 milestone set). Rethinking, I feel this could be duplicacy to setting the Milestone 2.0.0, but let's see if it is of any help to also have that label. If we think it's not needed, we can delete that label.