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, 1 insertions, 1 deletions

diff --git a/src/libstore/local-store.hh b/src/libstore/local-store.hh
index 780cc0f07..03bb0218d 100644
--- a/src/libstore/local-store.hh
+++ b/src/libstore/local-store.hh
@@ -280,7 +280,7 @@ private:
 
     void createUser(const std::string & userName, uid_t userId) override;
 
-    friend struct DerivationGoal;
+    friend struct LocalDerivationGoal;
     friend struct SubstitutionGoal;
 };