refactor: move go/build overlay to runtime/internal for minimal patching

- Moved full overlay file from runtime/_overlay/go/build/build.go to
  runtime/internal/go/build/build.go to make it clearer what's being overridden
- Updated runtime/overlay.go to embed from new location
- Created demo under _demo/go/gobuild to demonstrate go/build package working with llgo
- The overlay still contains the full file but is now organized under runtime/internal
  following the project's pattern for patched standard library packages

Fixes #1346

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Co-authored-by: luoliwoshang <luoliwoshang@users.noreply.github.com>

_demo/go/gobuild/demo.go

@@ -0,0 +1,17 @@
+package main
+
+import (
+	"fmt"
+	"go/build"
+	"runtime"
+)
+
+func main() {
+	fmt.Printf("runtime.Compiler = %q\n", runtime.Compiler)
+	pkg, err := build.Import("fmt", "", build.FindOnly)
+	if err != nil {
+		panic(err)
+	}
+	fmt.Printf("Package: %s\n", pkg.ImportPath)
+	fmt.Printf("Success! go/build works with llgo\n")
+}