`project` knows it's base dir

src/dds/project.cpp

     if (has_library_dirs(pf_dir_path)) {
         main_lib = library::from_directory(pf_dir_path);
     }
-    return project(std::move(main_lib), collect_submodules(pf_dir_path / "libs"));
+    return project(pf_dir_path, std::move(main_lib), collect_submodules(pf_dir_path / "libs"));
 }

src/dds/project.hpp

 namespace dds {
 
 class project {
+    fs::path               _root;
     std::optional<library> _main_lib;
     std::vector<library>   _submodules;
 
-    project(std::optional<library>&& ml, std::vector<library>&& mods)
-        : _main_lib(std::move(ml))
+    project(path_ref root, std::optional<library>&& ml, std::vector<library>&& mods)
+        : _root(root)
+        , _main_lib(std::move(ml))
         , _submodules(std::move(mods)) {}
 
 public:
         }
         return nullptr;
     }
+
+    path_ref root() const noexcept { return _root; }
 };
 
 }  // namespace dds