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>

runtime/overlay.go

@@ -22,7 +22,7 @@ var testing_testing_go124 string
 //go:embed _overlay/net/textproto/textproto.go
 var net_textproto string
 
-//go:embed _overlay/go/build/build.go
+//go:embed internal/go/build/build.go
 var go_build_build string
 
 var OverlayFiles = map[string]string{