loader: merge roots from both Go and TinyGo in a cached directory

This commit changes the way that packages are looked up. Instead of
working around the loader package by modifying the GOROOT variable for
specific packages, create a new GOROOT using symlinks. This GOROOT is
cached for the specified configuration (Go version, underlying GOROOT
path, TinyGo path, whether to override the syscall package).

This will also enable go module support in the future.

main.go

@@ -641,6 +641,36 @@ func getDefaultPort() (port string, err error) {
 	return d[0], nil
 }
 
+// runGoList runs the `go list` command but using the configuration used for
+// TinyGo.
+func runGoList(config *compileopts.Config, flagJSON, flagDeps bool, pkgs []string) error {
+	goroot, err := loader.GetCachedGoroot(config)
+	if err != nil {
+		return err
+	}
+	args := []string{"list"}
+	if flagJSON {
+		args = append(args, "-json")
+	}
+	if flagDeps {
+		args = append(args, "-deps")
+	}
+	if len(config.BuildTags()) != 0 {
+		args = append(args, "-tags", strings.Join(config.BuildTags(), " "))
+	}
+	args = append(args, pkgs...)
+	cgoEnabled := "0"
+	if config.CgoEnabled() {
+		cgoEnabled = "1"
+	}
+	cmd := exec.Command("go", args...)
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	cmd.Env = append(os.Environ(), "GOROOT="+goroot, "GOOS="+config.GOOS(), "GOARCH="+config.GOARCH(), "CGO_ENABLED="+cgoEnabled)
+	cmd.Run()
+	return nil
+}
+
 func usage() {
 	fmt.Fprintln(os.Stderr, "TinyGo is a Go compiler for small places.")
 	fmt.Fprintln(os.Stderr, "version:", version)
@@ -652,6 +682,7 @@ func usage() {
 	fmt.Fprintln(os.Stderr, "  flash: compile and flash to the device")
 	fmt.Fprintln(os.Stderr, "  gdb:   run/flash and immediately enter GDB")
 	fmt.Fprintln(os.Stderr, "  env:   list environment variables used during build")
+	fmt.Fprintln(os.Stderr, "  list:  run go list using the TinyGo root")
 	fmt.Fprintln(os.Stderr, "  clean: empty cache directory ("+goenv.Get("GOCACHE")+")")
 	fmt.Fprintln(os.Stderr, "  help:  print this help text")
 	fmt.Fprintln(os.Stderr, "\nflags:")
@@ -706,6 +737,13 @@ func handleCompilerError(err error) {
 }
 
 func main() {
+	if len(os.Args) < 2 {
+		fmt.Fprintln(os.Stderr, "No command-line arguments supplied.")
+		usage()
+		os.Exit(1)
+	}
+	command := os.Args[1]
+
 	outpath := flag.String("o", "", "output filename")
 	opt := flag.String("opt", "z", "optimization level: 0, 1, 2, s, z")
 	gc := flag.String("gc", "", "garbage collector to use (none, leaking, extalloc, conservative)")
@@ -726,12 +764,11 @@ func main() {
 	wasmAbi := flag.String("wasm-abi", "js", "WebAssembly ABI conventions: js (no i64 params) or generic")
 	heapSize := flag.String("heap-size", "1M", "default heap size in bytes (only supported by WebAssembly)")
 
-	if len(os.Args) < 2 {
-		fmt.Fprintln(os.Stderr, "No command-line arguments supplied.")
-		usage()
-		os.Exit(1)
+	var flagJSON, flagDeps *bool
+	if command == "list" {
+		flagJSON = flag.Bool("json", false, "print data in JSON format")
+		flagDeps = flag.Bool("deps", false, "")
 	}
-	command := os.Args[1]
 
 	// Early command processing, before commands are interpreted by the Go flag
 	// library.
@@ -895,6 +932,18 @@ func main() {
 		fmt.Printf("build tags:        %s\n", strings.Join(config.BuildTags(), " "))
 		fmt.Printf("garbage collector: %s\n", config.GC())
 		fmt.Printf("scheduler:         %s\n", config.Scheduler())
+	case "list":
+		config, err := builder.NewConfig(options)
+		if err != nil {
+			fmt.Fprintln(os.Stderr, err)
+			usage()
+			os.Exit(1)
+		}
+		err = runGoList(config, *flagJSON, *flagDeps, flag.Args())
+		if err != nil {
+			fmt.Fprintln(os.Stderr, "failed to run `go list`:", err)
+			os.Exit(1)
+		}
 	case "clean":
 		// remove cache directory
 		err := os.RemoveAll(goenv.Get("GOCACHE"))