[Ekapkgs] Have derivation helpers overwrite overrideAttrs

[jonringer]

In nixpkgs today, we have many mkDerivation "helpers" which expose a facade to mkDerivation. Instead of having overrideAttrs just affect the attrs which were passed to mkDerivation, I would instead prefer for the mkDerivation's overrideAttrs to be replaced by the downstream helper.

The goal is to make the interface which was used by an individual to be most reflective of what overrideAttrs would be targeting.

Specifically:

• mkDerivation should alias the current .overrideAttrs function to .overrideMkDerivationAttrs
  • overrideAttrs still exists
• "mkDerivation helpers" (e.g. buildPythonPackage) will overwrite .overrideAttrs with their own usage.
  • They can also expose their specific override attrs as well (E.g. overridePythonAttrs).