v5.6.5: Module Consistency

Module consistency (#58)

Breaking:

${defaultPackageName}.packages module option renamed to ${defaultPackageName}.packageDefinitions.replace

Depreciation warning for the above change added.

Non-Breaking, option additions for consistency:

In addition, the rest of the options to mirror categoryDefinitions module options have been added
${defaultPackageName}.packageDefinitions.merge
${defaultPackageName}.packageDefinitions.existing

nixos module gained the ability to set nixpkgs version or add overlays per user if desired, so that user options fully mirror system ones.