Fix handling of experimental features mid-parse

If we conditionally "declare" the argument, as we did before, based upon
weather the feature is enabled, commands like

    nix --experimental-features=foo ... --thing-gated-on-foo

won't work, because the experimental feature isn't enabled until *after*
we start parsing.

Instead, allow arguments to also be associated with experimental
features (just as we did for builtins and settings), and then the
command line parser will filter out the experimental ones.

Since the effects of arguments (handler functions) are performed right
away, we get the required behavior: earlier arguments can enable later
arguments enabled!

There is just one catch: we want to keep non-positional
flags...non-positional. So if

    nix --experimental-features=foo ... --thing-gated-on-foo

works, then

    nix --thing-gated-on-foo --experimental-features=foo ...

should also work.

This is not my favorite long-term solution, but for now this is
implemented by delaying the requirement of needed experimental features
until *after* all the arguments have been parsed.

src/libutil/args.hh

@@ -117,6 +117,8 @@ protected:
         Handler handler;
         std::function<void(size_t, std::string_view)> completer;
 
+        std::optional<ExperimentalFeature> experimentalFeature;
+
         static Flag mkHashTypeFlag(std::string && longName, HashType * ht);
         static Flag mkHashTypeOptFlag(std::string && longName, std::optional<HashType> * oht);
     };
@@ -188,6 +190,16 @@ public:
     friend class MultiCommand;
 
     MultiCommand * parent = nullptr;
+
+private:
+
+    /**
+     * Experimental features needed when parsing args. These are checked
+     * after flag parsing is completed in order to support enabling
+     * experimental features coming after the flag that needs the
+     * experimental feature.
+     */
+    std::set<ExperimentalFeature> flagExperimentalFeatures;
 };
 
 /* A command is an argument parser that can be executed by calling its
@@ -253,8 +265,6 @@ enum CompletionType {
 };
 extern CompletionType completionType;
 
-std::optional<std::string> needsCompletion(std::string_view s);
-
 void completePath(size_t, std::string_view prefix);
 
 void completeDir(size_t, std::string_view prefix);