message: Add find_installed() overload
diff: @@ -20,6 +20,11 @@ namespace vcpkg
  }
  const_iterator find(const std::string& name, const Triplet& target_triplet) const;
  iterator find(const std::string& name, const Triplet& target_triplet);
  +
  + const_iterator find_installed(const PackageSpec& spec) const
  + {
  + return find_installed(spec.name(), spec.target_triplet());
  + }
  const_iterator find_installed(const std::string& name, const Triplet& target_triplet) const;
  iterator insert(std::unique_ptr<StatusParagraph>);
  