tpl: Add TemplateFuncsNamespaceRegistry

As a first step to remove the hard ties between `tplimpl` and the different namespace packages.

The `lang` package is used as the first example use case.

See #3042

tpl/lang/init.go

@@ -0,0 +1,44 @@
+// Copyright 2017 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package lang
+
+import (
+	"github.com/spf13/hugo/deps"
+	"github.com/spf13/hugo/tpl/internal"
+)
+
+const name = "lang"
+
+func init() {
+	f := func(d *deps.Deps) *internal.TemplateFuncsNamespace {
+		ctx := New(d)
+
+		examples := [][2]string{
+			{},
+		}
+
+		return &internal.TemplateFuncsNamespace{
+			Name:    name,
+			Context: func() interface{} { return ctx },
+			Aliases: map[string]interface{}{
+				"i18n": ctx.Translate,
+				"T":    ctx.Translate,
+			},
+			Examples: examples,
+		}
+
+	}
+
+	internal.AddTemplateFuncsNamespace(f)
+}

tpl/lang/lang.go

@@ -31,6 +31,7 @@ type Namespace struct {
 }
 
 // Namespace returns a pointer to the current namespace instance.
+// TODO(bep) namespace remove this and other unused when done.
 func (ns *Namespace) Namespace() *Namespace { return ns }
 
 // Translate ...
@@ -42,8 +43,3 @@ func (ns *Namespace) Translate(id interface{}, args ...interface{}) (string, err
 
 	return ns.deps.Translate(sid, args...), nil
 }
-
-// T is an alias to Translate.
-func (ns *Namespace) T(id interface{}, args ...interface{}) (string, error) {
-	return ns.Translate(id, args...)
-}