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/libutil/include/nix/util/util.hh

@@ -242,6 +242,28 @@ std::string stripIndentation(std::string_view s);
  */
 std::pair<std::string_view, std::string_view> getLine(std::string_view s);
 
+/**
+ * Get a pointer to the contents of a `std::optional` if it is set, or a
+ * null pointer otherise.
+ *
+ * Const version.
+ */
+template<class T>
+const T * get(const std::optional<T> & opt)
+{
+    return opt ? &*opt : nullptr;
+}
+
+/**
+ * Non-const counterpart of `const T * get(const std::optional<T>)`.
+ * Takes a mutable reference, but returns a mutable pointer.
+ */
+template<class T>
+T * get(std::optional<T> & opt)
+{
+    return opt ? &*opt : nullptr;
+}
+
 /**
  * Get a value for the specified key from an associate container.
  */