Restore dynamic derivations!

This method does *not* create a new type of goal. We instead just make
`DerivationGoal` more sophisticated, which is much easier to do now that
`DerivationBuildingGoal` has been split from it (and so many fields are
gone, or or local variables instead).

This avoids the need for a secondarily trampoline goal that interacted
poorly with `addWantedOutputs`. That, I hope, will mean the bugs from
before do not reappear.

There may in fact be a reason to introduce such a trampoline in the
future, but it would only happen in conjunction with getting rid of
`addWantedOutputs`.

Restores the functionality (and tests) that was reverted in
f4f28cdd0e.

tests/functional/dyn-drv/build-built-drv.sh

@@ -18,4 +18,9 @@ clearStore
 
 drvDep=$(nix-instantiate ./text-hashed-output.nix -A producingDrv)
 
-expectStderr 1 nix build "${drvDep}^out^out" --no-link | grepQuiet "Building dynamic derivations in one shot is not yet implemented"
+# Store layer needs bugfix
+requireDaemonNewerThan "2.30pre20250515"
+
+out2=$(nix build "${drvDep}^out^out" --no-link)
+
+test $out1 == $out2

tests/functional/dyn-drv/dep-built-drv-2.sh

@@ -3,7 +3,7 @@
 source common.sh
 
 # Store layer needs bugfix
-requireDaemonNewerThan "2.27pre20250205"
+requireDaemonNewerThan "2.30pre20250515"
 
 TODO_NixOS # can't enable a sandbox feature easily
 
@@ -13,4 +13,4 @@ restartDaemon
 NIX_BIN_DIR="$(dirname "$(type -p nix)")"
 export NIX_BIN_DIR
 
-expectStderr 1 nix build -L --file ./non-trivial.nix --no-link | grepQuiet "Building dynamic derivations in one shot is not yet implemented"
+nix build -L --file ./non-trivial.nix --no-link

tests/functional/dyn-drv/dep-built-drv.sh

@@ -4,8 +4,11 @@ source common.sh
 
 out1=$(nix-build ./text-hashed-output.nix -A hello --no-out-link)
 
+# Store layer needs bugfix
+requireDaemonNewerThan "2.30pre20250515"
+
 clearStore
 
-expectStderr 1 nix-build ./text-hashed-output.nix -A wrapper --no-out-link | grepQuiet "Building dynamic derivations in one shot is not yet implemented"
+out2=$(nix-build ./text-hashed-output.nix -A wrapper --no-out-link)
 
-# diff -r $out1 $out2
+diff -r $out1 $out2

tests/functional/dyn-drv/failing-outer.sh

@@ -3,9 +3,7 @@
 source common.sh
 
 # Store layer needs bugfix
-requireDaemonNewerThan "2.27pre20250205"
-
-skipTest "dyn drv input scheduling had to be reverted for 2.27"
+requireDaemonNewerThan "2.30pre20250515"
 
 expected=100
 if [[ -v NIX_DAEMON_PACKAGE ]]; then expected=1; fi # work around the daemon not returning a 100 status correctly