ManifestRegistrar#finalize! avoids duplicates

This resolves a subtlety introduced by the fix in #286

`Container#finalize!` processes providers first, before moving on to
manifest registrations.

If a class injects something defined by a manifest file, and this class
is instantiated within a provider, this eager-loads the manifest key
before `finalize!` reaches the ManifestRegistrar.

Moving `ManifestRegistrar#call` to use `load` instead of `require`
introduced the possibility of double-registering a key.

Author: alassek

lib/dry/system/manifest_registrar.rb

@@ -26,8 +26,13 @@ def initialize(container)
 
       # @api private
       def finalize!
+        eager_loaded = container.keys.each_with_object(Set.new) do |key, set|
+          set << Identifier.new(key).root_key
+        end
+
         ::Dir[registrations_dir.join(RB_GLOB)].each do |file|
-          call(Identifier.new(File.basename(file, RB_EXT)))
+          ident = Identifier.new(File.basename(file, RB_EXT))
+          call(ident) unless eager_loaded.include?(ident.root_key)
         end
       end
 

spec/integration/container/providers/eager_loading_manifest_dependency_spec.rb

@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+RSpec.describe "Providers / Eager-loading a manifest-registered dependency" do
+  before :context do
+    @dir = make_tmp_directory
+
+    with_directory(@dir) do
+      write "system/registrations/deps.rb", <<~RUBY
+        Test::Container.namespace(:deps) do |container|
+          container.register(:thing) { Object.new }
+        end
+      RUBY
+    end
+  end
+
+  before do
+    root = @dir
+    Test::Container = Class.new(Dry::System::Container) do
+      configure do |config|
+        config.root = root
+      end
+
+      register_provider :my_provider do
+        start do
+          target["deps.thing"]
+        end
+      end
+    end
+  end
+
+  it "does not register the manifest dependency more than once" do
+    expect { Test::Container.finalize! }.not_to raise_error
+    expect(Test::Container["deps.thing"]).to be
+  end
+end