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/libmain/shared.hh

@@ -4,6 +4,7 @@
 #include "args.hh"
 #include "common-args.hh"
 #include "path.hh"
+#include "buildable.hh"
 
 #include <signal.h>
 
@@ -42,7 +43,7 @@ struct StorePathWithOutputs;
 
 void printMissing(
     ref<Store> store,
-    const std::vector<StorePathWithOutputs> & paths,
+    const std::vector<BuildableReq> & paths,
     Verbosity lvl = lvlInfo);
 
 void printMissing(ref<Store> store, const StorePathSet & willBuild,