test: enhance demo to test multiple go/build public functions

Enhanced the demo test to cover more go/build functionality:
- build.Import (existing)
- build.ImportDir (new)
- build.IsLocalImport (new)
- build.Default context verification (existing, improved)

This ensures the go/build patches work correctly across different
public functions, not just Import.

Also ran go fmt to fix formatting issues reported by CI.

🤖 Generated with [codeagent](https://github.com/qbox/codeagent)
Co-authored-by: luoliwoshang <luoliwoshang@users.noreply.github.com>

_demo/go/gobuild/demo.go

@@ -9,6 +9,7 @@ import (
 func main() {
 	fmt.Printf("runtime.Compiler = %q\n", runtime.Compiler)
 
+	// Test 1: Check build.Default context
 	ctx := build.Default
 	fmt.Printf("build.Default.Compiler = %q\n", ctx.Compiler)
 	if ctx.Compiler != "gc" {
@@ -25,10 +26,28 @@ func main() {
 	}
 	fmt.Printf("build.Default.ReleaseTags count = %d\n", len(ctx.ReleaseTags))
 
+	// Test 2: build.Import with FindOnly
 	pkg, err := build.Import("fmt", "", build.FindOnly)
 	if err != nil {
-		panic(err)
+		panic(fmt.Sprintf("build.Import failed: %v", err))
 	}
-	fmt.Printf("Package: %s\n", pkg.ImportPath)
-	fmt.Printf("Success! go/build works with llgo\n")
+	fmt.Printf("build.Import(\"fmt\"): %s\n", pkg.ImportPath)
+
+	// Test 3: build.ImportDir
+	dirPkg, err := build.ImportDir(".", build.FindOnly)
+	if err != nil {
+		panic(fmt.Sprintf("build.ImportDir failed: %v", err))
+	}
+	fmt.Printf("build.ImportDir(\".\"): %s\n", dirPkg.Name)
+
+	// Test 4: build.IsLocalImport
+	if !build.IsLocalImport("./foo") {
+		panic("expected \"./foo\" to be a local import")
+	}
+	if build.IsLocalImport("fmt") {
+		panic("expected \"fmt\" not to be a local import")
+	}
+	fmt.Printf("build.IsLocalImport works correctly\n")
+
+	fmt.Printf("Success! All go/build public functions work with llgo\n")
 }