internal/fetch: remove explicit goexperiment.jsonv2 enabling for encoding/json

Since goexperiment.jsonv2 is enabled by default in Go 1.27,
we don't need to explicitly enable it for "encoding/json" in
matchingFiles. It will be inherited from the toolchain defaults
when running on Go 1.27+.

This avoids incorrectly enabling it when processing encoding/json
package doc of older Go versions unnecessarily.

Updates golang/go#71488

Fixes golang/go#80325

Change-Id: I341de2f0571d7cc9c4045b806285bcede9d8853c
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/803341
kokoro-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ethan Lee <ethanalee@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

internal/fetch/load.go

diff --git a/internal/fetch/load.go b/internal/fetch/load.go
index b1437c4..74073ea 100644
--- a/internal/fetch/load.go
+++ b/internal/fetch/load.go
@@ -452,7 +452,7 @@
 	}
 
 	switch importPath {
-	case "encoding/json", "encoding/json/v2", "encoding/json/jsontext":
+	case "encoding/json/v2", "encoding/json/jsontext":
 		bctx.ToolTags = append(bctx.ToolTags, "goexperiment.jsonv2")
 	case "runtime/secret":
 		bctx.ToolTags = append(bctx.ToolTags, "goexperiment.runtimesecret")

internal/fetch/load_test.go

diff --git a/internal/fetch/load_test.go b/internal/fetch/load_test.go
index 36c7150..e2114b8 100644
--- a/internal/fetch/load_test.go
+++ b/internal/fetch/load_test.go
@@ -68,6 +68,13 @@
 		otherWant["other_jsonv2.go"] = []byte(otherJSONv2Body)
 	}
 
+	jsonWant := map[string][]byte{
+		"plain.go": []byte(plainGoBody),
+	}
+	if slices.Contains(build.Default.ToolTags, "goexperiment.jsonv2") {
+		jsonWant["json.go"] = []byte(jsonv2Body)
+	}
+
 	for _, test := range []struct {
 		name         string
 		goos, goarch string
@@ -115,10 +122,7 @@
 			goarch:     "amd64",
 			importPath: "encoding/json",
 			contents:   jsonv2Contents,
-			want: map[string][]byte{
-				"plain.go": []byte(plainGoBody),
-				"json.go":  []byte(jsonv2Body),
-			},
+			want:       jsonWant,
 		},
 		{
 			name:       "jsonv2",