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/parsed-derivations.cc

@@ -100,7 +100,7 @@ static nlohmann::json pathInfoToJSON(Store & store, const StorePathSet & storePa
 
 nlohmann::json::object_t StructuredAttrs::prepareStructuredAttrs(
     Store & store,
-    const DerivationOptions & drvOptions,
+    const DerivationOptions<StorePath> & drvOptions,
     const StorePathSet & inputPaths,
     const DerivationOutputs & outputs) const
 {
@@ -114,8 +114,8 @@ nlohmann::json::object_t StructuredAttrs::prepareStructuredAttrs(
     json["outputs"] = std::move(outputsJson);
 
     /* Handle exportReferencesGraph. */
-    for (auto & [key, storePaths] : drvOptions.getParsedExportReferencesGraph(store)) {
-        json[key] = pathInfoToJSON(store, store.exportReferences(storePaths, storePaths));
+    for (auto & [key, storePaths] : drvOptions.exportReferencesGraph) {
+        json[key] = pathInfoToJSON(store, store.exportReferences(storePaths, inputPaths));
     }
 
     return json;