search: implement link_requires

We have to track whether a component is a link only component, and the
only add the link flags.

TODO:
  - need a test for a case where we add a component as link-only, then
    need it as a full requires

src/cps/search.cpp

@@ -420,14 +420,16 @@ namespace cps::search {
                         self_defaults = true;
                         const std::vector<std::string> & defs = node->data.package.default_components.value();
                         std::transform(defs.begin(), defs.end(), std::back_insert_iterator(node->data.components),
-                                       [](std::string name) { return Component{std::move(name)}; });
+                                       [link_only](std::string name) {
+                                           return Component{std::move(name), link_only};
+                                       });
                     }
                     for (auto && comp : self->second.components) {
                         if (processed.find(comp) != processed.end()) {
                             continue;
                         }
-                        self_requires.emplace_back(Component{comp});
-                        node->data.components.emplace_back(Component{comp});
+                        self_requires.emplace_back(Component{comp, link_only});
+                        node->data.components.emplace_back(Component{comp, link_only});
                     }
                 }
             }