internal/rosa: extra inputs in alternative path

This works around particularly unwieldy build systems.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/rosa/azalea/evaluate.go

@@ -142,6 +142,8 @@ var (
 	IdentInputs = unique.Make(Ident("inputs"))
 	// IdentRuntime has the same semantics as [IdentInputs].
 	IdentRuntime = unique.Make(Ident("runtime"))
+	// IdentExtra has the same semantics as [IdentInputs].
+	IdentExtra = unique.Make(Ident("extra"))
 	// IdentSource is a special argument in a package declaration where an
 	// assignment of a [Val] with a single [Ident] passes through the evaluator.
 	IdentSource = unique.Make(Ident("source"))
@@ -291,6 +293,7 @@ func evaluateAny(d PF, s []Frame, expr, rp any) bool {
 				for _, special := range [...]unique.Handle[Ident]{
 					IdentInputs,
 					IdentRuntime,
+					IdentExtra,
 				} {
 					if slices.Contains(names, special) {
 						if len(names) != 1 || len(arg.V) != 1 || arg.R {

internal/rosa/state.go

@@ -926,7 +926,7 @@ func (ctx *evalContext) pf(
 		sourceA any
 		helper  Helper
 
-		inputs, runtimes azalea.Array
+		inputs, runtimes, extra azalea.Array
 	)
 	if err = args.Apply(map[unique.Handle[azalea.Ident]]any{
 		k("description"): &meta.Description,
@@ -952,14 +952,17 @@ func (ctx *evalContext) pf(
 		k("exec"):     &helper,
 		k("inputs"):   &inputs,
 		k("runtime"):  &runtimes,
+		k("extra"):    &extra,
 	}); err != nil {
 		return
 	}
-	var inputsH P
+	var inputsH, extraH P
 	if inputsH, err = toHandles(inputs); err != nil {
 		return
 	} else if meta.Dependencies, err = toHandles(runtimes); err != nil {
 		return
+	} else if extraH, err = toHandles(extra); err != nil {
+		return
 	}
 
 	meta.ID = int(anitya)
@@ -981,6 +984,15 @@ func (ctx *evalContext) pf(
 		})
 	}
 
+	for _, h := range extraH {
+		_, a := ctx.t.MustLoad(h)
+		attr.Paths = append(attr.Paths, pkg.Path(
+			AbsUsrSrc.Append("extra", h.String()),
+			attr.Writable,
+			a,
+		))
+	}
+
 	for _, pair := range files {
 		var pathname *check.Absolute
 		if pathname, err = check.NewAbs(pair[0]); err != nil {