Rework evaluator SingleDerivedPath infra

`EvalState::mkSingleDerivedPathString` previously contained its own
inverse (printing, rather than parsing) in order to validate what was
parsed. Now that is pulled out into its own separate function:
`EvalState::coerceToSingleDerivedPath`.

In additional that pulled out logic is deduplicated with
`EvalState::mkOutputString` via `EvalState::mkOutputStringRaw`, which is
itself deduplicated (and generalized) with
`DownstreamPlaceholder::mkOutputStringRaw`.

All these changes make the unit tests simpler.

(We would ideally write more unit tests for `mkSingleDerivedPathString`
`coerceToSingleDerivedPath` directly, but we cannot yet do that because
the IO in reading the store path won't work when the dummy store cannot
hold anything. Someday we'll have a proper in-memory store which will
work for this.)

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

src/libexpr/primops.cc

@@ -156,8 +156,10 @@ static void mkOutputString(
 {
     state.mkOutputString(
         attrs.alloc(o.first),
-        drvPath,
-        o.first,
+        SingleDerivedPath::Built {
+            .drvPath = makeConstantStorePathRef(drvPath),
+            .output = o.first,
+        },
         o.second.path(*state.store, Derivation::nameFromPath(drvPath), o.first));
 }