Backend refactor and clean

[fridgepoet]

What this PR does / why we need it:

• Return error earlier in processQuery
• Replace variable function declarations with just functions: we are not currently replacing these functions and it may be best if we avoided that in the future
• Export aggContainer: used in a different package for testing this upcoming PR. I hate to do a change for a test but maybe it is a hint that things are not organized quite right. Or maybe I have not written the test in the right place. I'd like to keep the upcoming PR focused on the feature changes and this kind of change creates unnecessary noise in it. Let me know if we'd prefer it elsewhere!
• Clean unused index in searchRequestBuilder

Which issue(s) this PR fixes:

Preparation while working on #280

Special notes for your reviewer:
This was and can be tested locally with our team's plugin provisioning set up and with the Lucene queries going through the backend: cherry-pick the commit from this branch main...backend-flow