Eliminate the "store" global variable

Also, move a few free-standing functions into StoreAPI and Derivation.

Also, introduce a non-nullable smart pointer, ref<T>, which is just a
wrapper around std::shared_ptr ensuring that the pointer is never
null. (For reference-counted values, this is better than passing a
"T&", because the latter doesn't maintain the refcount. Usually, the
caller will have a shared_ptr keeping the value alive, but that's not
always the case, e.g., when passing a reference to a std::thread via
std::bind.)

src/libstore/download.hh

@@ -1,6 +1,7 @@
 #pragma once
 
 #include "types.hh"
+
 #include <string>
 
 namespace nix {
@@ -18,9 +19,11 @@ struct DownloadResult
     string data, etag;
 };
 
+class StoreAPI;
+
 DownloadResult downloadFile(string url, const DownloadOptions & options);
 
-Path downloadFileCached(const string & url, bool unpack);
+Path downloadFileCached(ref<StoreAPI> store, const string & url, bool unpack);
 
 MakeError(DownloadError, Error)