Fix resolve_reposession_conflicts not resolving anything

The code still iterated over $orig.FLATTENABLE_HASH as needed in NQP but which
doesn't actually work in Perl 6. It also lacked recursive merging of stashes,
i.e. with Foo::Bar::Baz and Foo::Bar::Qux we would lose the first one as we
found a "Bar" in $current and assumed it was the same.

Both issues can be fixed by using merge-symbols which gives us the same
semantics as during compilation, with the exception of our scoped functions.
No idea how they could even conflict when they are in different packages like
NativeCall's &mangle_cpp_symbol. But as it seems to work and unification of
compilation and module loading semantics can only be a good thing, we adopt the
"latest wins" semantics of resolve_reposession_conflicts in merge_globals for
now.

src/Perl6/ModuleLoader.nqp

@@ -161,6 +161,10 @@ class Perl6::ModuleLoader does Perl6::ModuleLoaderVMConfig {
                     self.merge_globals($_.value.WHO, ($target){$sym}.WHO);
                     ($target){$sym} := $_.value;
                 }
+                elsif nqp::eqat($_.key, '&', 0) {
+                    # "Latest wins" semantics for functions
+                    ($target){$sym} := $_.value;
+                }
                 else {
                     nqp::die("P6M Merging GLOBAL symbols failed: duplicate definition of symbol $sym");
                 }

src/core/CompUnit/RepositoryRegistry.pm

@@ -199,11 +199,7 @@ RAKUDO_MODULE_DEBUG("Looking in $spec for $name")
             # If it's a Stash in conflict, we make sure any original entries get
             # appropriately copied.
             if $orig.HOW.name($orig) eq 'Stash' {
-                for $orig.FLATTENABLE_HASH() {
-                    if !nqp::existskey($current, $_.key) || nqp::eqat($_.key, '&', 0) {
-                        $current{$_.key} := $_.value;
-                    }
-                }
+                $current.merge-symbols($orig);
             }
             # We could complain about anything else, and may in the future; for
             # now, we let it pass by with "latest wins" semantics.