Offload symbol lookup onto Raku code

And get rid of `find_exception` method. I don't see areason why
metamodel code should implement what Raku does better.

Added `&sym_lookup` local of Perl6::Metamodel::Configuration and a
method to register a lookup routine. Now `throw_or_die` uses
registration of the routine as a flag to start using core exception
classes instead of plain `nqp::die`.

src/Perl6/Metamodel/Configuration.nqp

@@ -39,43 +39,20 @@ class Perl6::Metamodel::Configuration {
     }
     method role_to_role_applier_type() { $role_to_role_applier_type }
 
-    my $X_package := nqp::null();
-    method set_X_package($X) {
-        $X_package := $X;
-    }
-    method find_exception($exception) {
-        my $ex_type := nqp::null();
-        unless nqp::isnull($X_package) {
-            my @parts := nqp::split('::', $exception);
-            # If the first part of the long name is not X then pretend there is no such exception. Which is, actually
-            # and most likely, is true.
-            if nqp::iseq_s(nqp::shift(@parts), 'X') {
-                my $who := $X_package.WHO;
-                while +@parts {
-                    my $name := nqp::shift(@parts);
-                    if $who.EXISTS-KEY($name) {
-                        if +@parts {
-                            $who := $who.AT-KEY($name).WHO;
-                        }
-                        else {
-                            $ex_type := $who.AT-KEY($name);
-                        }
-                    }
-                    else {
-                        # Signal the loop end
-                        @parts := nqp::list();
-                    }
-                }
-            }
-        }
-        $ex_type
+    my &sym_lookup := nqp::null();
+    method set_sym_lookup_routine(&slr) {
+        &sym_lookup := &slr;
     }
     method throw_or_die($exception, $die_message, *@pos, *%named) {
-        my $ex_type := self.find_exception($exception);
-        if nqp::isnull($ex_type) {
+        if nqp::isnull(&sym_lookup) {
             nqp::die($die_message)
         }
         else {
+            # When &sym_lookup is registered we do have all core exception classes declared. Therefore we can use
+            # use &sym_lookup safely. If it fails to find a symbol then fully legit X::NoSuchSymbol will be thrown.
+            my $ex_type := &sym_lookup(nqp::hllizefor($exception, 'Raku'));
+            # HLLize all named arguments for exception constructor. Note that if an exception attribute is Bool the the
+            # caller of this method is responsible for using nqp::hllboolfor to produce a valid Bool instance.
             my %hll_named;
             for %named {
                 %hll_named{nqp::iterkey_s($_)} := nqp::hllizefor(nqp::iterval($_), 'Raku');

src/core.c/Exception.pm6

@@ -3314,7 +3314,9 @@ my class Exceptions::JSON {
     }
 }
 
-# Provide means of accessing any X:: exception to the Metamodel.
-Metamodel::Configuration.set_X_package(X);
+# Provide Metamodel::Configuration with symbol lookup routine. We do it here because throw_or_die method learn about
+# availability of all exception classes based on this registration. OTOH, it is better to provide them as soon as
+# possible as this might improve diagnostics of CORE.setting compilation failures.
+Metamodel::Configuration.set_sym_lookup_routine( -> $sym is raw { ::($sym) } );
 
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