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/libstore/content-address.hh

@@ -5,6 +5,7 @@
 #include "hash.hh"
 #include "path.hh"
 #include "comparator.hh"
+#include "variant-wrapper.hh"
 
 namespace nix {
 
@@ -71,11 +72,7 @@ struct ContentAddressMethod
 
     GENERATE_CMP(ContentAddressMethod, me->raw);
 
-    /* The moral equivalent of `using Raw::Raw;` */
-    ContentAddressMethod(auto &&... arg)
-        : raw(std::forward<decltype(arg)>(arg)...)
-    { }
-
+    MAKE_WRAPPER_CONSTRUCTOR(ContentAddressMethod);
 
     /**
      * Parse the prefix tag which indicates how the files
@@ -252,10 +249,7 @@ struct ContentAddressWithReferences
 
     GENERATE_CMP(ContentAddressWithReferences, me->raw);
 
-    /* The moral equivalent of `using Raw::Raw;` */
-    ContentAddressWithReferences(auto &&... arg)
-        : raw(std::forward<decltype(arg)>(arg)...)
-    { }
+    MAKE_WRAPPER_CONSTRUCTOR(ContentAddressWithReferences);
 
     /**
      * Create a `ContentAddressWithReferences` from a mere