Fixing #7479

Types converted:

- `NixStringContextElem`
- `OutputsSpec`
- `ExtendedOutputsSpec`
- `DerivationOutput`
- `DerivationType`

Existing ones mostly conforming the pattern cleaned up:

- `ContentAddressMethod`
- `ContentAddressWithReferences`

The `DerivationGoal::derivationType` field had a bogus initialization,
now caught, so I made it `std::optional`. I think #8829 can make it
non-optional again because it will ensure we always have the derivation
when we construct a `DerivationGoal`.

See that issue (#7479) for details on the general goal.

`git grep 'Raw::Raw'` indicates the two types I didn't yet convert
`DerivedPath` and `BuiltPath` (and their `Single` variants) . This is
because @roberth and I (can't find issue right now...) plan on reworking
them somewhat, so I didn't want to churn them more just yet.

Co-authored-by: Eelco Dolstra <edolstra@gmail.com>

src/libutil/variant-wrapper.hh

@@ -0,0 +1,30 @@
+#pragma once
+///@file
+
+// not used, but will be used by callers
+#include <variant>
+
+/**
+ * Force the default versions of all constructors (copy, move, copy
+ * assignment).
+ */
+#define FORCE_DEFAULT_CONSTRUCTORS(CLASS_NAME) \
+    CLASS_NAME(const CLASS_NAME &) = default; \
+    CLASS_NAME(CLASS_NAME &) = default; \
+    CLASS_NAME(CLASS_NAME &&) = default; \
+    \
+    CLASS_NAME & operator =(const CLASS_NAME &) = default; \
+    CLASS_NAME & operator =(CLASS_NAME &) = default;
+
+/**
+ * Make a wrapper constructor. All args are forwarded to the
+ * construction of the "raw" field. (Which we assume is the only one.)
+ *
+ * The moral equivalent of `using Raw::Raw;`
+ */
+#define MAKE_WRAPPER_CONSTRUCTOR(CLASS_NAME) \
+    FORCE_DEFAULT_CONSTRUCTORS(CLASS_NAME) \
+    \
+    CLASS_NAME(auto &&... arg) \
+        : raw(std::forward<decltype(arg)>(arg)...) \
+    { }