Make the Derived Path family of types inductive for dynamic derivations

We want to be able to write down `foo.drv^bar.drv^baz`:
`foo.drv^bar.drv` is the dynamic derivation (since it is itself a
derivation output, `bar.drv` from `foo.drv`).

To that end, we create `Single{Derivation,BuiltPath}` types, that are
very similar except instead of having multiple outputs (in a set or
map), they have a single one. This is for everything to the left of the
rightmost `^`.

`NixStringContextElem` has an analogous change, and now can reuse
`SingleDerivedPath` at the top level. In fact, if we ever get rid of
`DrvDeep`, `NixStringContextElem` could be replaced with
`SingleDerivedPath` entirely!

Important note: some JSON formats have changed.

We already can *produce* dynamic derivations, but we can't refer to them
directly. Today, we can merely express building or example at the top
imperatively over time by building `foo.drv^bar.drv`, and then with a
second nix invocation doing `<result-from-first>^baz`, but this is not
declarative. The ethos of Nix of being able to write down the full plan
everything you want to do, and then execute than plan with a single
command, and for that we need the new inductive form of these types.

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>
Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

tests/dyn-drv/build-built-drv.sh

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+
+source common.sh
+
+# In the corresponding nix file, we have two derivations: the first, named `hello`,
+# is a normal recursive derivation, while the second, named dependent, has the
+# new outputHashMode "text". Note that in "dependent", we don't refer to the
+# build output of `hello`, but only to the path of the drv file. For this reason,
+# we only need to:
+#
+# - instantiate `hello`
+# - build `producingDrv`
+# - check that the path of the output coincides with that of the original derivation
+
+out1=$(nix build -f ./text-hashed-output.nix hello --no-link)
+
+clearStore
+
+drvDep=$(nix-instantiate ./text-hashed-output.nix -A producingDrv)
+
+expectStderr 1 nix build "${drvDep}^out^out" --no-link | grepQuiet "Building dynamic derivations in one shot is not yet implemented"