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/meson.go

@@ -72,9 +72,7 @@ func (*MesonHelper) name(name, version string) string {
 }
 
 // extra returns hardcoded meson runtime dependencies.
-func (*MesonHelper) extra(int) []PArtifact {
-	return []PArtifact{Meson}
-}
+func (*MesonHelper) extra(int) P { return P{Meson} }
 
 // wantsChmod returns false.
 func (*MesonHelper) wantsChmod() bool { return false }