ca derivations are what we're really after, but dynamic derivations
must also go because they depend on ca derivations. we can't easily
implement dynamic derivations any other way, so we remove them too.
impure derivations build on the content-addressed infrastructure in
ways we cannot easily detangle, so they too must go for time being.
see #815
Change-Id: If61371736dfd89cc71a1b2ae5a005757c3cb9484
(cherry picked from commit d8e2f53d07)
eldritch horrors
37a570bd40