Split out the puthe path/filepath functions into common/paths

So they can be used from the config package without cyclic troubles.

Updates #8654

create/content.go

@@ -21,6 +21,8 @@ import (
 	"path/filepath"
 	"strings"
 
+	"github.com/gohugoio/hugo/common/paths"
+
 	"github.com/pkg/errors"
 
 	"github.com/gohugoio/hugo/common/hexec"
@@ -39,7 +41,7 @@ import (
 func NewContent(
 	sites *hugolib.HugoSites, kind, targetPath string) error {
 	targetPath = filepath.Clean(targetPath)
-	ext := helpers.Ext(targetPath)
+	ext := paths.Ext(targetPath)
 	ps := sites.PathSpec
 	archetypeFs := ps.BaseFs.SourceFilesystems.Archetypes.Fs
 	sourceFs := ps.Fs.Source

create/content_template_handler.go

@@ -20,6 +20,8 @@ import (
 	"strings"
 	"time"
 
+	"github.com/gohugoio/hugo/common/paths"
+
 	"github.com/pkg/errors"
 
 	"github.com/gohugoio/hugo/helpers"
@@ -129,7 +131,7 @@ func executeArcheTypeAsTemplate(s *hugolib.Site, name, kind, targetPath, archety
 
 	// Reuse the Hugo template setup to get the template funcs properly set up.
 	templateHandler := s.Deps.Tmpl().(tpl.TemplateManager)
-	templateName := helpers.Filename(archetypeFilename)
+	templateName := paths.Filename(archetypeFilename)
 	if err := templateHandler.AddTemplate("_text/"+templateName, string(archetypeTemplate)); err != nil {
 		return nil, errors.Wrapf(err, "Failed to parse archetype file %q:", archetypeFilename)
 	}