hydra-module.nix: Give types to options

Also, rename the "service.hydra.hydra" option to "service.hydra.package" to be consistent with NixOS option naming conventions.

Created by  Eelco Dolstra  on November 6, 2013
7XOH2DGH3W4B7BJWCXDP673R5R7TQGLCNTHJT7IZUSBVQWYWJUHQC
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