Make compiler look for exceptions in setting only.

If people declared a package X, it could hide the X namespace for
exceptions, so the compiler could not find its exception types to
throw for things like syntax errors. This has led to all manner of
confusion. So, now we only look for those in the current setting.

src/Perl6/World.nqp

@@ -3076,7 +3076,7 @@ class Perl6::World is HLL::World {
     # Finds a symbol that has a known value at compile time from the
     # perspective of the current scope. Checks for lexicals, then if
     # that fails tries package lookup.
-    method find_symbol(@name) {
+    method find_symbol(@name, :$setting-only) {
         # Make sure it's not an empty name.
         unless +@name { nqp::die("Cannot look up empty name"); }
 
@@ -3089,7 +3089,7 @@ class Perl6::World is HLL::World {
         # scopes.
         if +@name == 1 {
             my $final_name := @name[0];
-            my int $i := +@!BLOCKS;
+            my int $i := $setting-only ?? 1 !! +@!BLOCKS;
             while $i > 0 {
                 $i := $i - 1;
                 my %sym := @!BLOCKS[$i].symbol($final_name);
@@ -3105,7 +3105,7 @@ class Perl6::World is HLL::World {
         my $result := $*GLOBALish;
         if +@name >= 2 {
             my $first := @name[0];
-            my int $i := +@!BLOCKS;
+            my int $i := $setting-only ?? 1 !! +@!BLOCKS;
             while $i > 0 {
                 $i := $i - 1;
                 my %sym := @!BLOCKS[$i].symbol($first);
@@ -3385,8 +3385,10 @@ class Perl6::World is HLL::World {
                 nqp::print("Error while constructing error object:");
                 nqp::say($_);
             };
-            $ex := self.find_symbol(nqp::islist($ex_type) ?? $ex_type !! nqp::split('::', $ex_type));
-            my $x_comp := self.find_symbol(['X', 'Comp']);
+            $ex := self.find_symbol(
+                nqp::islist($ex_type) ?? $ex_type !! nqp::split('::', $ex_type),
+                :setting-only);
+            my $x_comp := self.find_symbol(['X', 'Comp'], :setting-only);
             unless nqp::istype($ex, $x_comp) {
                 $ex := $ex.HOW.mixin($ex, $x_comp);
             }