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/libexpr/primops/context.cc

@@ -51,13 +51,13 @@ static void prim_unsafeDiscardOutputDependency(EvalState & state, const PosIdx p
 
     NixStringContext context2;
     for (auto && c : context) {
-        if (auto * ptr = std::get_if<NixStringContextElem::DrvDeep>(&c)) {
+        if (auto * ptr = std::get_if<NixStringContextElem::DrvDeep>(&c.raw)) {
             context2.emplace(NixStringContextElem::Opaque {
                 .path = ptr->drvPath
             });
         } else {
             /* Can reuse original item */
-            context2.emplace(std::move(c));
+            context2.emplace(std::move(c).raw);
         }
     }
 
@@ -114,7 +114,7 @@ static void prim_getContext(EvalState & state, const PosIdx pos, Value * * args,
             [&](NixStringContextElem::Opaque && o) {
                 contextInfos[std::move(o.path)].path = true;
             },
-        }, ((NixStringContextElem &&) i).raw());
+        }, ((NixStringContextElem &&) i).raw);
     }
 
     auto attrs = state.buildBindings(contextInfos.size());