internal/rosa: expose in-place behaviour in generic helper

This change also combines the createDir and wantsDir methods, and replaces the non-inplace target of the generic helper with a deterministic path.

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

internal/rosa/package/perl.az

@@ -59,10 +59,12 @@ package perl-Module-Build {
 		checksum = "ZKxEFG4hE1rqZt52zBL2LRZBMkYzhjb5-cTBXcsyA52EbPeeYyVxU176yAea8-Di";
 	};
 
+	enterSource = true;
 	writable = true;
 	chmod = true;
 
 	exec = generic {
+		inPlace = true;
 		build = `
 perl Build.PL --prefix=/system
 ./Build build`;