Expand the ACE base template lookup pattern

The previous implementation didn't easily support the use case "I want one base template for the single pages, another for the rest".

The new lookup order is:

1. <current-path>/<template-name>-baseof.ace, e.g. list-baseof.ace
2. <current-path>/baseof.ace
3. _default/<template-name>-baseof.ace, e.g. list-baseof.ace.
4. _default/baseof.ace

tpl/template.go

@@ -15,6 +15,7 @@ package tpl
 
 import (
 	"bytes"
+	"fmt"
 	"github.com/eknkc/amber"
 	bp "github.com/spf13/hugo/bufferpool"
 	"github.com/spf13/hugo/helpers"
@@ -279,14 +280,26 @@ func (t *GoHTMLTemplate) loadTemplates(absPath string, prefix string) {
 					return err
 				}
 				if needsBase {
-					// Look for the base first in the current path, then in _default.
-					p := filepath.Join(filepath.Dir(path), baseAceFilename)
-					if ok, err := helpers.Exists(p, hugofs.OsFs); err == nil && ok {
-						baseTemplatePath = p
-					} else {
-						p := filepath.Join(absPath, "_default", baseAceFilename)
-						if ok, err := helpers.Exists(p, hugofs.OsFs); err == nil && ok {
-							baseTemplatePath = p
+
+					// Look for base template in the follwing order:
+					//   1. <current-path>/<template-name>-baseof.ace, e.g. list-baseof.ace.
+					//   2. <current-path>/baseof.ace
+					//   3. _default/<template-name>-baseof.ace, e.g. list-baseof.ace.
+					//   4. _default/baseof.ace
+
+					currBaseAceFilename := fmt.Sprintf("%s-%s", helpers.Filename(path), baseAceFilename)
+					templateDir := filepath.Dir(path)
+
+					pathsToCheck := []string{
+						filepath.Join(templateDir, currBaseAceFilename),
+						filepath.Join(templateDir, baseAceFilename),
+						filepath.Join(absPath, "_default", currBaseAceFilename),
+						filepath.Join(absPath, "_default", baseAceFilename)}
+
+					for _, pathToCheck := range pathsToCheck {
+						if ok, err := helpers.Exists(pathToCheck, hugofs.OsFs); err == nil && ok {
+							baseTemplatePath = pathToCheck
+							break
 						}
 					}
 				}