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/libcmd/installables.cc

@@ -459,7 +459,7 @@ Installables SourceExprCommand::parseInstallables(
             result.push_back(
                 make_ref<InstallableAttrPath>(
                     InstallableAttrPath::parse(
-                        state, *this, vFile, prefix, extendedOutputsSpec)));
+                        state, *this, vFile, std::move(prefix), std::move(extendedOutputsSpec))));
         }
 
     } else {
@@ -475,7 +475,7 @@ Installables SourceExprCommand::parseInstallables(
             if (prefix.find('/') != std::string::npos) {
                 try {
                     result.push_back(make_ref<InstallableDerivedPath>(
-                        InstallableDerivedPath::parse(store, prefix, extendedOutputsSpec)));
+                        InstallableDerivedPath::parse(store, prefix, extendedOutputsSpec.raw)));
                     continue;
                 } catch (BadStorePath &) {
                 } catch (...) {
@@ -491,7 +491,7 @@ Installables SourceExprCommand::parseInstallables(
                         getEvalState(),
                         std::move(flakeRef),
                         fragment,
-                        extendedOutputsSpec,
+                        std::move(extendedOutputsSpec),
                         getDefaultFlakeAttrPaths(),
                         getDefaultFlakeAttrPathPrefixes(),
                         lockFlags));