devx: Standardized backend code layout

Created by: tsenart

Sourcegraph's backend code has a lack of strong conventions in its structure. This has been identified as a drag on on-boarding time of new engineers, or of existing engineers into new areas of code.

Before we even think of using the same application architecture for all domains (e.g. MVC), we could start by trying to simplify and standardize the package structure to make it more intuitive.

Prior proposal in this area: https://docs.google.com/document/d/1WyMlVtrwe2qqOGhPYgnvFxUYqllz2t-CVH3sD4qMKTE/edit#