Move build logic for the core hydra package into its own default.nix.

This allows users to build hydra using a normal mkDerivation, package overrides, etc.

Created by  Shea Levy  on April 11, 2017
UBPWXGKFBD6VMQCCUE625I4NLLRNVD7BSJ6QD2XF7Z7A7T5BI5XAC
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