Remove the "locked" flag from the fetcher cache

This also reworks the Mercurial fetcher (which was still using the
old cache interface) to have two distinct cache mappings:

* A ref-to-rev mapping, which is store-independent.
* A rev-to-store-path mapping.

src/libfetchers/tarball.cc

@@ -99,8 +99,7 @@ DownloadFileResult downloadFile(
             *store,
             inAttrs,
             infoAttrs,
-            *storePath,
-            false);
+            *storePath);
     }
 
     return {