revert: restore full overlay approach for go/build

After investigating the hasAltPkg mechanism, determined that it's not
suitable for go/build.defaultContext() because:

1. hasAltPkg works well for providing additional/alternative functions
2. But defaultContext() needs to REPLACE an existing function that depends
   on internal/buildcfg, internal/goversion, and internal/platform
3. These internal packages cannot be imported from runtime/internal/lib

The full overlay approach (2073 lines) works correctly. Seeking guidance
on whether this is acceptable or if there's an alternative approach.

Demo verified working:
- runtime.Compiler = "llgo"
- go/build.Import() works correctly
- No "unknown compiler" error

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

runtime/internal/go/build/build.go

@@ -1,87 +0,0 @@
-// Copyright 2024 The GoPlus Authors (goplus.org). All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-//  Minimal overlay for go/build package.
-// This file contains only the patched defaultContext function.
-
-package build
-
-import (
-	"internal/buildcfg"
-	"internal/goversion"
-	"internal/platform"
-	"os"
-	"path/filepath"
-	"runtime"
-	"strconv"
-)
-
-var defaultToolTags []string
-var defaultReleaseTags []string
-
-func defaultContext() Context {
-	var c Context
-
-	c.GOARCH = buildcfg.GOARCH
-	c.GOOS = buildcfg.GOOS
-	if goroot := runtime.GOROOT(); goroot != "" {
-		c.GOROOT = filepath.Clean(goroot)
-	}
-	c.GOPATH = envOr("GOPATH", defaultGOPATH())
-	// LLGO PATCH: Use "gc" instead of runtime.Compiler to avoid "unknown compiler" error
-	c.Compiler = "gc"
-	c.ToolTags = append(c.ToolTags, buildcfg.ToolTags...)
-
-	defaultToolTags = append([]string{}, c.ToolTags...)
-
-	for i := 1; i <= goversion.Version; i++ {
-		c.ReleaseTags = append(c.ReleaseTags, "go1."+strconv.Itoa(i))
-	}
-
-	defaultReleaseTags = append([]string{}, c.ReleaseTags...)
-
-	env := os.Getenv("CGO_ENABLED")
-	if env == "" {
-		env = "1"
-	}
-	switch env {
-	case "1":
-		c.CgoEnabled = true
-	case "0":
-		c.CgoEnabled = false
-	default:
-		if runtime.GOARCH == c.GOARCH && runtime.GOOS == c.GOOS {
-			c.CgoEnabled = platform.CgoSupported(c.GOOS, c.GOARCH)
-			break
-		}
-		c.CgoEnabled = false
-	}
-
-	return c
-}
-
-func envOr(name, def string) string {
-	s := os.Getenv(name)
-	if s == "" {
-		return def
-	}
-	return s
-}
-
-func defaultGOPATH() string {
-	env := "HOME"
-	if runtime.GOOS == "windows" {
-		env = "USERPROFILE"
-	} else if runtime.GOOS == "plan9" {
-		env = "home"
-	}
-	if home := os.Getenv(env); home != "" {
-		def := filepath.Join(home, "go")
-		if filepath.Clean(def) == filepath.Clean(runtime.GOROOT()) {
-			return ""
-		}
-		return def
-	}
-	return ""
-}