Let push_dir raise if given an anonymous root namespace

Anonymous root namespaces are not supported, Zeitwerk assumes classes and
modules have regular names in a few places.

However, if you pass an anonymous root namespace right now the library crashes
in some random internal spot. It is better to tell the user at the boundary and
in a controlled way. Also, this allows us to work internally knowing that root
namespaces in `roots` have a name for sure.

lib/zeitwerk/loader/config.rb

@@ -5,6 +5,7 @@
 
 module Zeitwerk::Loader::Config
   extend Zeitwerk::Internal
+  include Zeitwerk::RealModName
 
   # @sig #camelize
   attr_accessor :inflector
@@ -113,6 +114,10 @@ def push_dir(path, namespace: Object)
       raise Zeitwerk::Error, "#{namespace.inspect} is not a class or module object, should be"
     end
 
+    unless real_mod_name(namespace)
+      raise Zeitwerk::Error, "root namespaces cannot be anonymous"
+    end
+
     abspath = File.expand_path(path)
     if dir?(abspath)
       raise_if_conflicting_directory(abspath)

test/lib/zeitwerk/test_push_dir.rb

@@ -49,4 +49,9 @@ def check_dirs
     e = assert_raises(Zeitwerk::Error) { loader.push_dir(".", namespace: :foo) }
     assert_equal ":foo is not a class or module object, should be", e.message
   end
+
+  test "raises if the namespace is anonymous" do
+    e = assert_raises(Zeitwerk::Error) { loader.push_dir(".", namespace: Module.new) }
+    assert_equal "root namespaces cannot be anonymous", e.message
+  end
 end