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>
Co-authored-by: Eelco Dolstra <edolstra@gmail.com>

src/libstore/misc.cc

@@ -225,11 +225,12 @@ MissingPaths Store::queryMissing(const std::vector<DerivedPath> & targets)
                         return;
 
                     auto drv = make_ref<Derivation>(derivationFromPath(drvPath));
-                    DerivationOptions drvOptions;
+                    DerivationOptions<SingleDerivedPath> drvOptions;
                     try {
                         // FIXME: this is a lot of work just to get the value
                         // of `allowSubstitutes`.
-                        drvOptions = DerivationOptions::fromStructuredAttrs(drv->env, drv->structuredAttrs);
+                        drvOptions = derivationOptionsFromStructuredAttrs(
+                            *this, drv->inputDrvs, drv->env, get(drv->structuredAttrs));
                     } catch (Error & e) {
                         e.addTrace({}, "while parsing derivation '%s'", printStorePath(drvPath));
                         throw;