Update Package names

[ajlacey]

Packages in go are mapped onto their directories (https://go.dev/blog/package-names#package-paths). Keeping this organization maintains logical grouping of code and expresses relationships with subpackages.

Packages that need renaming:

• possibly all packages within backend/handlers could be moved to the handlers package, but definitely need to be renamed

Packages that need regrouping:

• ~~backend/tests~~ complete in #317
• backend/util

Packages that look good:

• backend/logging
• backend/middleware
• backend/migration (may still benefit from renaming or regrouping)
• backend/models
• backend/seed
• backend/server (may still benefit from renaming or regrouping)
• backend/setup