Add a way to get all the outputs of a derivation with their label

Generalize `queryDerivationOutputNames` and `queryDerivationOutputs` by
adding a `queryDerivationOutputMap` that returns the map
`outputName=>outputPath`

(not that this is not equivalent to merging the results of
`queryDerivationOutputs` and `queryDerivationOutputNames` as sets don't
preserve the order, so we would end up with an incorrect mapping).

squash! Add a way to get all the outputs of a derivation with their label

Rename StorePathMap to OutputPathMap

src/libstore/store-api.cc

@@ -242,6 +242,16 @@ bool Store::PathInfoCacheValue::isKnownNow()
     return std::chrono::steady_clock::now() < time_point + ttl;
 }
 
+StorePathSet Store::queryDerivationOutputs(const StorePath & path)
+{
+    auto outputMap = this->queryDerivationOutputMap(path);
+    StorePathSet outputPaths;
+    for (auto & i: outputMap) {
+        outputPaths.emplace(std::move(i.second));
+    }
+    return outputPaths;
+}
+
 bool Store::isValidPath(const StorePath & storePath)
 {
     std::string hashPart(storePath.hashPart());