Cache reflect.MethodByName

The isolated benchmark for the function is obviously much faster:

```bash
name                old time/op    new time/op    delta
GetMethodByName-10    1.21µs ± 7%    0.23µs ± 5%   -81.42%  (p=0.029 n=4+4)

name                old alloc/op   new alloc/op   delta
GetMethodByName-10      680B ± 0%        0B       -100.00%  (p=0.029 n=4+4)

name                old allocs/op  new allocs/op  delta
GetMethodByName-10      20.0 ± 0%       0.0       -100.00%  (p=0.029 n=4+4)
```

But more pleasing is the overall performance looking at the site benchmarks:

```bash
name                                      old time/op    new time/op    delta
SiteNew/Regular_Bundle_with_image-10        6.25ms ± 2%    6.10ms ± 2%     ~     (p=0.057 n=4+4)
SiteNew/Regular_Bundle_with_JSON_file-10    6.30ms ± 2%    5.66ms ±11%     ~     (p=0.057 n=4+4)
SiteNew/Regular_Tags_and_categories-10      22.2ms ± 2%    17.4ms ± 1%  -21.88%  (p=0.029 n=4+4)
SiteNew/Regular_Canonify_URLs-10             108ms ± 0%     107ms ± 0%   -1.20%  (p=0.029 n=4+4)
SiteNew/Regular_Deep_content_tree-10        36.1ms ± 1%    33.8ms ± 1%   -6.44%  (p=0.029 n=4+4)
SiteNew/Regular_TOML_front_matter-10        24.9ms ± 1%    22.6ms ± 1%   -9.30%  (p=0.029 n=4+4)
SiteNew/Regular_Many_HTML_templates-10      17.9ms ± 1%    16.7ms ± 1%   -6.43%  (p=0.029 n=4+4)
SiteNew/Regular_Page_collections-10         23.3ms ± 1%    22.0ms ± 0%   -5.58%  (p=0.029 n=4+4)
SiteNew/Regular_List_terms-10               8.00ms ± 1%    7.63ms ± 0%   -4.62%  (p=0.029 n=4+4)

name                                      old alloc/op   new alloc/op   delta
SiteNew/Regular_Bundle_with_image-10        2.10MB ± 0%    2.07MB ± 0%   -1.46%  (p=0.029 n=4+4)
SiteNew/Regular_Bundle_with_JSON_file-10    1.88MB ± 0%    1.85MB ± 0%   -1.76%  (p=0.029 n=4+4)
SiteNew/Regular_Tags_and_categories-10      13.5MB ± 0%    11.6MB ± 0%  -13.99%  (p=0.029 n=4+4)
SiteNew/Regular_Canonify_URLs-10            96.1MB ± 0%    95.8MB ± 0%   -0.40%  (p=0.029 n=4+4)
SiteNew/Regular_Deep_content_tree-10        28.4MB ± 0%    27.3MB ± 0%   -3.83%  (p=0.029 n=4+4)
SiteNew/Regular_TOML_front_matter-10        16.9MB ± 0%    15.1MB ± 0%  -10.58%  (p=0.029 n=4+4)
SiteNew/Regular_Many_HTML_templates-10      8.98MB ± 0%    8.44MB ± 0%   -6.04%  (p=0.029 n=4+4)
SiteNew/Regular_Page_collections-10         17.1MB ± 0%    16.5MB ± 0%   -3.91%  (p=0.029 n=4+4)
SiteNew/Regular_List_terms-10               3.92MB ± 0%    3.72MB ± 0%   -5.03%  (p=0.029 n=4+4)

name                                      old allocs/op  new allocs/op  delta
SiteNew/Regular_Bundle_with_image-10         25.8k ± 0%     24.9k ± 0%   -3.49%  (p=0.029 n=4+4)
SiteNew/Regular_Bundle_with_JSON_file-10     25.8k ± 0%     24.9k ± 0%   -3.49%  (p=0.029 n=4+4)
SiteNew/Regular_Tags_and_categories-10        288k ± 0%      233k ± 0%  -18.90%  (p=0.029 n=4+4)
SiteNew/Regular_Canonify_URLs-10              375k ± 0%      364k ± 0%   -2.80%  (p=0.029 n=4+4)
SiteNew/Regular_Deep_content_tree-10          314k ± 0%      283k ± 0%   -9.77%  (p=0.029 n=4+4)
SiteNew/Regular_TOML_front_matter-10          302k ± 0%      252k ± 0%  -16.55%  (p=0.029 n=4+4)
SiteNew/Regular_Many_HTML_templates-10        133k ± 0%      117k ± 0%  -11.81%  (p=0.029 n=4+4)
SiteNew/Regular_Page_collections-10           202k ± 0%      183k ± 0%   -9.55%  (p=0.029 n=4+4)
SiteNew/Regular_List_terms-10                55.6k ± 0%     49.8k ± 0%  -10.40%  (p=0.029 n=4+4)
```

Thanks to @quasilyte for the suggestion.

Fixes 9386

2 files changed, 8 insertions, 4 deletions

diff --git a/resources/page/pagegroup.go b/resources/page/pagegroup.go
index e07efa7ca..4c0adb68b 100644
--- a/resources/page/pagegroup.go
+++ b/resources/page/pagegroup.go
@@ -24,6 +24,7 @@ import (
 	"github.com/spf13/cast"
 
 	"github.com/gohugoio/hugo/common/collections"
+	"github.com/gohugoio/hugo/common/hreflect"
 	"github.com/gohugoio/hugo/compare"
 
 	"github.com/gohugoio/hugo/resources/resource"
@@ -112,8 +113,9 @@ func (p Pages) GroupBy(key string, order ...string) (PagesGroup, error) {
 	}
 
 	var ft interface{}
-	m, ok := pagePtrType.MethodByName(key)
-	if ok {
+	index := hreflect.GetMethodIndexByName(pagePtrType, key)
+	if index != -1 {
+		m := pagePtrType.Method(index)
 		if m.Type.NumOut() == 0 || m.Type.NumOut() > 2 {
 			return nil, errors.New(key + " is a Page method but you can't use it with GroupBy")
 		}
@@ -125,6 +127,7 @@ func (p Pages) GroupBy(key string, order ...string) (PagesGroup, error) {
 		}
 		ft = m
 	} else {
+		var ok bool
 		ft, ok = pagePtrType.Elem().FieldByName(key)
 		if !ok {
 			return nil, errors.New(key + " is neither a field nor a method of Page")
@@ -146,7 +149,7 @@ func (p Pages) GroupBy(key string, order ...string) (PagesGroup, error) {
 		case reflect.StructField:
 			fv = ppv.Elem().FieldByName(key)
 		case reflect.Method:
-			fv = ppv.MethodByName(key).Call([]reflect.Value{})[0]
+			fv = hreflect.GetMethodByName(ppv, key).Call([]reflect.Value{})[0]
 		}
 		if !fv.IsValid() {
 			continue
diff --git a/resources/postpub/postpub.go b/resources/postpub/postpub.go
index c11dda577..f1012b7da 100644
--- a/resources/postpub/postpub.go
+++ b/resources/postpub/postpub.go
@@ -21,6 +21,7 @@ import (
 
 	"github.com/spf13/cast"
 
+	"github.com/gohugoio/hugo/common/hreflect"
 	"github.com/gohugoio/hugo/common/maps"
 	"github.com/gohugoio/hugo/media"
 	"github.com/gohugoio/hugo/resources/resource"
@@ -125,7 +126,7 @@ func (r *PostPublishResource) fieldToString(receiver interface{}, path string) s
 	default:
 		v := receiverv.FieldByName(fieldname)
 		if !v.IsValid() {
-			method := receiverv.MethodByName(fieldname)
+			method := hreflect.GetMethodByName(receiverv, fieldname)
 			if method.IsValid() {
 				vals := method.Call(nil)
 				if len(vals) > 0 {