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/path.hh

@@ -62,6 +62,7 @@ public:
 
 typedef std::set<StorePath> StorePathSet;
 typedef std::vector<StorePath> StorePaths;
+typedef std::map<string, StorePath> OutputPathMap;
 
 /* Extension of derivations in the Nix store. */
 const std::string drvExtension = ".drv";