Add function queryPathFromHashPart()

To implement binary caches efficiently, Hydra needs to be able to map
the hash part of a store path (e.g. "gbg...zr7") to the full store
path (e.g. "/nix/store/gbg...kzr7-subversion-1.7.5").  (The binary
cache mechanism uses hash parts as a key for looking up store paths to
ensure privacy.)  However, doing a search in the Nix store for
/nix/store/<hash>* is expensive since it requires reading the entire
directory.  queryPathFromHashPart() prevents this by doing a cheap
database lookup.

perl/lib/Nix/Store.pm

@@ -14,6 +14,7 @@ our @EXPORT_OK = ( @{ $EXPORT_TAGS{'all'} } );
 
 our @EXPORT = qw(
     isValidPath queryReferences queryPathInfo queryDeriver queryPathHash
+    queryPathFromHashPart
     topoSortPaths computeFSClosure followLinksToStorePath exportPaths
     hashPath hashFile hashString
     addToStore makeFixedOutputPath