Revert "Change the default "from" to "Raku""

This reverts commit 6354c91.

Hmmm... looks like this breaks zef, will look more in depth later

src/core.c/CompUnit.pm6

@@ -21,7 +21,7 @@ class CompUnit {
     # (if known).
     has Distribution $.distribution;
 
-    my constant $default-from = 'Raku';
+    my $default-from = 'Perl6';
 
     method new(CompUnit:U:
       Str                  :$short-name is copy,