Split {,local-}derivation-goal.{cc,hh}

This separates the scheduling logic (including simple hook pathway) from
the local-store needing code.

This should be the final split for now. I'm reasonably happy with how
it's turning out, even before I'm done moving code into
`local-derivation-goal`. Benefits:

1. This will help "witness" that the hook case is indeed a lot simpler,
   and also compensate for the increased complexity that comes from
   content-addressed derivation outputs.

2. It also moves us ever so slightly towards a world where we could use
   off-the-shelf storage or sandboxing, since `local-derivation-goal`
   would be gutted in those cases, but `derivation-goal` should remain
   nearly the same.

The new `#if 0` in the new files will be deleted in the following
commit. I keep it here so if it turns out more stuff can be moved over,
it's easy to do so in a way that preserves ordering --- and thus
prevents conflicts.

N.B.
```sh
git diff HEAD^^ --color-moved --find-copies-harder --patience --stat
```
makes nicer output.

1 files changed, 9 insertions, 5 deletions

diff --git a/src/libstore/build/worker.cc b/src/libstore/build/worker.cc
index 2f13aa885..b2223c3b6 100644
--- a/src/libstore/build/worker.cc
+++ b/src/libstore/build/worker.cc
@@ -1,7 +1,7 @@
 #include "machines.hh"
 #include "worker.hh"
 #include "substitution-goal.hh"
-#include "derivation-goal.hh"
+#include "local-derivation-goal.hh"
 #include "hook-instance.hh"
 
 #include <poll.h>
@@ -59,8 +59,10 @@ std::shared_ptr<DerivationGoal> Worker::makeDerivationGoalCommon(
 std::shared_ptr<DerivationGoal> Worker::makeDerivationGoal(const StorePath & drvPath,
     const StringSet & wantedOutputs, BuildMode buildMode)
 {
-    return makeDerivationGoalCommon(drvPath, wantedOutputs, [&]() {
-        return std::make_shared<DerivationGoal>(drvPath, wantedOutputs, *this, buildMode);
+    return makeDerivationGoalCommon(drvPath, wantedOutputs, [&]() -> std::shared_ptr<DerivationGoal> {
+        return !dynamic_cast<LocalStore *>(&store)
+            ? std::make_shared</* */DerivationGoal>(drvPath, wantedOutputs, *this, buildMode)
+            : std::make_shared<LocalDerivationGoal>(drvPath, wantedOutputs, *this, buildMode);
     });
 }
 
@@ -68,8 +70,10 @@ std::shared_ptr<DerivationGoal> Worker::makeDerivationGoal(const StorePath & drv
 std::shared_ptr<DerivationGoal> Worker::makeBasicDerivationGoal(const StorePath & drvPath,
     const BasicDerivation & drv, const StringSet & wantedOutputs, BuildMode buildMode)
 {
-    return makeDerivationGoalCommon(drvPath, wantedOutputs, [&]() {
-        return std::make_shared<DerivationGoal>(drvPath, drv, wantedOutputs, *this, buildMode);
+    return makeDerivationGoalCommon(drvPath, wantedOutputs, [&]() -> std::shared_ptr<DerivationGoal> {
+        return !dynamic_cast<LocalStore *>(&store)
+            ? std::make_shared</* */DerivationGoal>(drvPath, drv, wantedOutputs, *this, buildMode)
+            : std::make_shared<LocalDerivationGoal>(drvPath, drv, wantedOutputs, *this, buildMode);
     });
 }