refactor(build-image): Do not assemble image layers in Nix

This is the first step towards a more granular build process where
some of the build responsibility moves into the server component.

Rather than assembling all layers inside of Nix, it will only create
the symlink forest and return information about the runtime paths
required by the image.

The server is then responsible for grouping these paths into layers,
and assembling the layers themselves.

Relates to #50.

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