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/libcmd/installables.cc

@@ -468,10 +468,10 @@ std::vector<InstallableValue::DerivationInfo> InstallableAttrPath::toDerivations
 
     std::vector<DerivationInfo> res;
     for (auto & drvInfo : drvInfos) {
-        res.push_back({
-            state->store->parseStorePath(drvInfo.queryDrvPath()),
-            drvInfo.queryOutputName()
-        });
+        auto drvPath = drvInfo.queryDrvPath();
+        if (!drvPath)
+            throw Error("'%s' is not a derivation", what());
+        res.push_back({ *drvPath, drvInfo.queryOutputName() });
     }
 
     return res;