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>

src/libstore/legacy-ssh-store.cc

@@ -358,6 +358,9 @@ public:
                 [&](const StorePath & drvPath) {
                     throw Error("wanted to fetch '%s' but the legacy ssh protocol doesn't support merely substituting drv files via the build paths command. It would build them instead. Try using ssh-ng://", printStorePath(drvPath));
                 },
+                [&](std::monostate) {
+                    throw Error("wanted build derivation that is itself a build product, but the legacy ssh protocol doesn't support that. Try using ssh-ng://");
+                },
             }, sOrDrvPath);
         }
         conn->to << ss;