[WIP] [Nexus] Initial restructure for packaging

[brockdyer03]

Proposed changes

Change structure of Nexus to move towards real python packaging (Issue #5623)

This involves a lot, but the first step here is to just change the import structure of Nexus to be relative rather than absolute.

What type(s) of changes does this code introduce?

• Other: Restructuring

Does this introduce a breaking change?

• Yes

What systems has this change been tested on?

Partially on a laptop

Checklist

• • [ ] I have read the pull request guidance and develop docs
• • [ ] This PR is up to date with the current state of 'develop'
• • [ ] Code added or changed in the PR has been clang-formatted
• • [ ] This PR adds tests to cover any new code, or to catch a bug that is being fixed
• • [ ] Documentation has been added (if appropriate)