Use BuildableReq for buildPaths and ensurePath

This avoids an ambiguity where the `StorePathWithOutputs { drvPath, {}
}` could mean "build `brvPath`" or "substitute `drvPath`" depending on
context.

It also brings the internals closer in line to the new CLI, by
generalizing the `Buildable` type is used there and makes that
distinction already.

In doing so, relegate `StorePathWithOutputs` to being a type just for
backwards compatibility (CLI and RPC).

src/libstore/store-api.hh

@@ -2,7 +2,7 @@
 
 #include "realisation.hh"
 #include "path.hh"
-#include "path-with-outputs.hh"
+#include "buildable.hh"
 #include "hash.hh"
 #include "content-address.hh"
 #include "serialise.hh"
@@ -494,7 +494,7 @@ public:
        recursively building any sub-derivations. For inputs that are
        not derivations, substitute them. */
     virtual void buildPaths(
-        const std::vector<StorePathWithOutputs> & paths,
+        const std::vector<BuildableReq> & paths,
         BuildMode buildMode = bmNormal);
 
     /* Build a single non-materialized derivation (i.e. not from an
@@ -656,7 +656,7 @@ public:
     /* Given a set of paths that are to be built, return the set of
        derivations that will be built, and the set of output paths
        that will be substituted. */
-    virtual void queryMissing(const std::vector<StorePathWithOutputs> & targets,
+    virtual void queryMissing(const std::vector<BuildableReq> & targets,
         StorePathSet & willBuild, StorePathSet & willSubstitute, StorePathSet & unknown,
         uint64_t & downloadSize, uint64_t & narSize);