Parse deriving paths in DerivationOptions

This is an example of "Parse, don't validate" principle [1].

Before, we had a number of `StringSet`s in `DerivationOptions` that
were not *actually* allowed to be arbitrary sets of strings. Instead,
each set member had to be one of:

- a store path

- a CA "downstream placeholder"

- an output name

Only later, in the code that checks outputs, would these strings be
further parsed to match these cases. (Actually, only 2 by that point,
because the placeholders must be rewritten away by then.)

Now, we fully parse everything up front, and have an "honest" data type
that reflects these invariants:

- store paths are parsed, stored as (opaque) deriving paths

- CA "downstream placeholders" are rewritten to the output deriving
  paths they denote

- output names are the only arbitrary strings left

Since the first two cases both become deriving paths, that leaves us
with a `std::variant<SingleDerivedPath, String>` data type, which we use
in our sets instead.

Getting rid of placeholders is especially nice because we are replacing
them with something much more internally-structured / transparent.

[1]: https://lexi-lambda.github.io/blog/2019/11/05/parse-don-t-validate/

Co-authored-by: Sergei Zimmerman <sergei@zimmerman.foo>

tests/functional/check-refs.sh

@@ -64,5 +64,5 @@ fi
 if isDaemonNewer "2.28pre20241225"; then
     # test12 should fail (syntactically invalid).
     expectStderr 1 nix-build -vvv -o "$RESULT" check-refs.nix -A test12 >"$TEST_ROOT/test12.stderr"
-    grepQuiet -F "output check for 'lib' contains an illegal reference specifier 'dev', expected store path or output name (one of [lib, out])" < "$TEST_ROOT/test12.stderr"
+    grepQuiet -F "output check for 'lib' contains output name 'dev', but this is not a valid output of this derivation. (Valid outputs are [lib, out].)" < "$TEST_ROOT/test12.stderr"
 fi