Central location for pkg catalog db file

src/dds/catalog/catalog.cpp

 #include <dds/error/errors.hpp>
 #include <dds/solve/solve.hpp>
 #include <dds/util/log.hpp>
+#include <dds/util/paths.hpp>
 
 #include <json5/parse_data.hpp>
 #include <neo/assert.hpp>
 
 }  // namespace
 
+fs::path catalog::default_path() noexcept { return dds_data_dir() / "catalog.db"; }
+
 catalog catalog::open(const std::string& db_path) {
     if (db_path != ":memory:") {
         auto pardir = fs::weakly_canonical(db_path).parent_path();

src/dds/catalog/catalog.hpp

     static catalog open(const std::string& db_path);
     static catalog open(path_ref db_path) { return open(db_path.string()); }
 
+    static fs::path default_path() noexcept;
+
     void                        store(const package_info& info);
     std::optional<package_info> get(const package_id& id) const noexcept;