object-store: migrate alternates struct and functions from cache.h

Migrate the struct alternate_object_database and all its related
functions to the object store as these functions are easier found in
that header. The migration is just a verbatim copy, no need to
include the object store header at any C file, because cache.h includes
repository.h which in turn includes the object-store.h

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

tmp-objdir.c

@@ -6,6 +6,7 @@
 #include "strbuf.h"
 #include "argv-array.h"
 #include "quote.h"
+#include "object-store.h"
 
 struct tmp_objdir {
 	struct strbuf path;