Revert "Revert "Revert "Adapt scheduler to work with dynamic derivations"""

The bug reappeared after all, and the fix introduced a different bug. We
want to release 2.27 imminently so there is no time to do a proper fix,
which appears to require a larger reworking. Hopefully we will have it
for 2.28, however.

This reverts commit c98525235f.

src/libstore/derived-path-map.hh

@@ -21,11 +21,8 @@ namespace nix {
  *
  * @param V A type to instantiate for each output. It should probably
  * should be an "optional" type so not every interior node has to have a
- * value. For example, the scheduler uses
- * `DerivedPathMap<std::weak_ptr<DerivationCreationAndRealisationGoal>>` to
- * remember which goals correspond to which outputs. `* const Something`
- * or `std::optional<Something>` would also be good choices for
- * "optional" types.
+ * value. `* const Something` or `std::optional<Something>` would be
+ * good choices for "optional" types.
  */
 template<typename V>
 struct DerivedPathMap {