go/doc: reuse import name logic for examples

Examples resolved imports based on just the path name,
but this doesn't work for go-name or modules on v2+.

Updates #12794
Fixes #45110
Fixes #56740
Fixes #62059

Change-Id: I8c71b1e5311df04bccbdf319d759d3176a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/728280
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>

src/go/doc/example.go

@@ -11,7 +11,6 @@ import (
 	"go/ast"
 	"go/token"
 	"internal/lazyregexp"
-	"path"
 	"slices"
 	"strconv"
 	"strings"
@@ -221,7 +220,7 @@ func playExample(file *ast.File, f *ast.FuncDecl) *ast.File {
 			// because the package syscall/js is not available in the playground.
 			return nil
 		}
-		n := path.Base(p)
+		n := assumedPackageName(p)
 		if s.Name != nil {
 			n = s.Name.Name
 			switch n {

src/go/doc/testdata/examples/major_version.go

@@ -0,0 +1,14 @@
+package foo_test
+
+import (
+	"example.com/foo/v3"
+	"example.com/go-bar"
+)
+
+func Example() {
+	foo.Print("hello")
+	bar.Print("world")
+	// Output:
+	// hello
+	// world
+}

src/go/doc/testdata/examples/major_version.golden

@@ -0,0 +1,15 @@
+-- .Play --
+package main
+
+import (
+	"example.com/foo/v3"
+	"example.com/go-bar"
+)
+
+func main() {
+	foo.Print("hello")
+	bar.Print("world")
+}
+-- .Output --
+hello
+world