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/build.go

@@ -22,6 +22,7 @@ var hasAltPkg = map[string]none{
 	"crypto/sha256":            {},
 	"crypto/sha512":            {},
 	"crypto/subtle":            {},
+	"go/build":                 {},
 	"go/parser":                {},
 	"hash/crc32":               {},
 	"hash/maphash":             {},