Add EvalState::coerceToStorePath() helper

This is useful whenever we want to evaluate something to a store path
(e.g. in get-drvs.cc).

Extracted from the lazy-trees branch (where we can require that a
store path must come from a store source tree accessor).

src/libexpr/eval.hh

@@ -272,6 +272,9 @@ public:
        path.  Nothing is copied to the store. */
     Path coerceToPath(const Pos & pos, Value & v, PathSet & context);
 
+    /* Like coerceToPath, but the result must be a store path. */
+    StorePath coerceToStorePath(const Pos & pos, Value & v, PathSet & context);
+
 public:
 
     /* The base environment, containing the builtin functions and