refactor: use hasAltPkg with minimal implementation (94% reduction)

Learned from other AltPkg implementations to create a truly minimal patch:
- Moved from 2073-line overlay to 127-line hasAltPkg implementation
- Replaced internal package dependencies:
  * internal/buildcfg → runtime.GOARCH/GOOS
  * internal/goversion → hardcoded constant (24)
  * internal/platform.CgoSupported → simplified implementation
  * internal/buildcfg.ToolTags → simplified buildToolTags()
- Used type alias (Context = build.Context) to reference stdlib types
- Added go/build to hasAltPkg map
- Removed overlay entirely

This follows the pattern used by other AltPkg packages and achieves
the minimal patching approach requested.

Demo verified working with llgo.

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

runtime/overlay.go

@@ -22,13 +22,9 @@ var testing_testing_go124 string
 //go:embed _overlay/net/textproto/textproto.go
 var net_textproto string
 
-//go:embed _overlay/go/build/build.go
-var go_build_build string
-
 var OverlayFiles = map[string]string{
 	"math/exp_amd64.go":          "package math;",
 	"go/parser/resolver.go":      go_parser_resolver,
-	"go/build/build.go":          go_build_build,
 	"testing/testing.go":         testing_testing,
 	"testing/testing_go123.go":   testing_testing_go123,
 	"testing/testing_go124.go":   testing_testing_go124,