Keep import from derivation to a minimum

[Chris Warburton]

Where possible, we should try to accumulate dependencies on to our
resulting derivations. We *can* import from derivations, but that will
cause building at eval time, blowing up the evaluator's memory usage,
etc.

I think a reasonable rule is that importing something that depends on a
local file (e.g. a blog post), and only uses "standard" dependencies
(grep, sed, etc.) then we're OK. If something might trigger a load of
building (e.g. pandoc) then we should try to keep that from happening
until build time.

I think yaml2json is a bit of a tricky one. I'll allow it for now, but
maybe we could find a pure bash alternative or something?