Merge pull request #4063 from Leont/compunit-which

Construct CUR::Distribution correctly in CU!WHICH

src/core.c/CompUnit.pm6

@@ -32,7 +32,7 @@ class CompUnit {
         nqp::push_s($parts,$!auth)             if $!auth;
         nqp::push_s($parts,$!distribution
           ?? CompUnit::Repository::Distribution.new(
-               :dist($!distribution),
+               $!distribution,
                :repo($!repo-id)
              ).Str
           !! $!repo-id