Add file name to file-private metaclass types during codegen (#15505)

File-private types with the same name get collapsed into a single type during codegen, which messes up any dispatch over them: only one of their type IDs is considered, and the other is assumed to be unreachable.
Adding the file name as namespace keeps them apart.

spec/compiler/codegen/class_spec.cr

@@ -659,6 +659,38 @@ describe "Code gen: class" do
       )).to_string.should eq("Baz")
   end
 
+  it "does not combine types with same name but different file scopes (#15503)" do
+    run(<<-CRYSTAL, Int32, filename: "foo.cr").should eq(11)
+      module Foo
+        def self.foo
+          1
+        end
+      end
+
+      alias Bar = Foo
+
+      {% Bar %} # forces immediate resolution of `Bar`
+
+      private module Foo
+        def self.foo
+          10
+        end
+      end
+
+      module Baz
+        def self.foo
+          100
+        end
+      end
+
+      def foo(x)
+        x.foo
+      end
+
+      foo(Foo || Baz) &+ foo(Bar || Baz)
+      CRYSTAL
+  end
+
   it "builds generic class bug" do
     codegen(%(
       abstract class Base

src/compiler/crystal/types.cr

@@ -2935,6 +2935,12 @@ module Crystal
     end
 
     def to_s_with_options(io : IO, skip_union_parens : Bool = false, generic_args : Bool = true, codegen : Bool = false) : Nil
+      if codegen
+        if (namespace = instance_type.namespace).is_a?(FileModule)
+          namespace.to_s_with_options(io, generic_args: false, codegen: codegen)
+          io << "::"
+        end
+      end
       io << @name
     end