internal/rosa: use type P in helper interface

This is easier to type and serialises correctly.

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

internal/rosa/rosa.go

@@ -409,7 +409,7 @@ type Helper interface {
 	// name returns the value passed to the name argument of [Toolchain.New].
 	name(name, version string) string
 	// extra returns helper-specific dependencies.
-	extra(flag int) []PArtifact
+	extra(flag int) P
 
 	// wantsChmod returns whether the source directory should be made writable.
 	wantsChmod() bool