libutil: return a source from readFile

don't consume a sink, return a source instead. the only reason to not do
this is a very slight reduction in dynamic allocations, but since we are
going to *at least* do disk io that will not be a lot of overhead anyway

Change-Id: Iae2f879ec64c3c3ac1d5310eeb6a85e696d4614a

src/libstore/binary-cache-store.cc

@@ -385,7 +385,7 @@ StorePath BinaryCacheStore::addToStore(
     if (method == FileIngestionMethod::Recursive) {
         dumpPath(srcPath, sink, filter);
     } else {
-        readFile(srcPath, sink);
+        readFileSource(srcPath)->drainInto(sink);
     }
     auto h = sink.finish().first;