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/eval-cache.cc

@@ -604,7 +604,7 @@ string_t AttrCursor::getStringWithContext()
                         [&](const NixStringContextElem::Opaque & o) -> const StorePath & {
                             return o.path;
                         },
-                    }, c.raw());
+                    }, c.raw);
                     if (!root->state.store->isValidPath(path)) {
                         valid = false;
                         break;