only dynamic derivations could produce a non-opaque drvPath. since
dynamic derivations are no longer supported we can have drvPath be
opaque at all times, simplifying downstream code significantly and
making quite a few methods unnecessary. discardOutputPath was only
called on drvPath members anyway and thus reduces to a copy, other
operations at the very least are no longer recursive. some vestige
of dynamic derivations remains in DerivedPathMap though (for now).
Change-Id: Ifb4ad53a3c67800be5a62540068c8279d4ae0046
eldritch horrors
5f723e96e6