loader: rewrite/refactor much of the code to use go list directly

There were a few problems with the go/packages package. While it is more
or less designed for our purpose, it didn't work quite well as it didn't
provide access to indirectly imported packages (most importantly the
runtime package). This led to a workaround that sometimes broke
`tinygo test`.

This PR contains a number of related changes:

  * It uses `go list` directly to retrieve the list of packages/files to
    compile, instead of relying on the go/packages package.
  * It replaces our custom TestMain replace code with the standard code
    for running tests (generated by `go list`).
  * It adds a dummy runtime/pprof package and modifies the testing
    package, to get tests to run again with the code generated by
    `go list`.

main.go

@@ -126,12 +126,6 @@ func Test(pkgName string, options *compileopts.Options) error {
 		return err
 	}
 
-	// Add test build tag. This is incorrect: `go test` only looks at the
-	// _test.go file suffix but does not add the test build tag in the process.
-	// However, it's a simple fix right now.
-	// For details: https://github.com/golang/go/issues/21360
-	config.Target.BuildTags = append(config.Target.BuildTags, "test")
-
 	return builder.Build(pkgName, ".elf", config, func(tmppath string) error {
 		cmd := exec.Command(tmppath)
 		cmd.Stdout = os.Stdout
@@ -648,36 +642,6 @@ 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:", goenv.Version)
@@ -751,10 +715,16 @@ func printCompilerError(logln func(...interface{}), err error) {
 			}
 		}
 	case loader.Errors:
-		logln("#", err.Pkg.PkgPath)
+		logln("#", err.Pkg.ImportPath)
 		for _, err := range err.Errs {
 			printCompilerError(logln, err)
 		}
+	case loader.Error:
+		logln(err.Err.Error())
+		logln("package", err.ImportStack[0])
+		for _, pkgPath := range err.ImportStack[1:] {
+			logln("\timports", pkgPath)
+		}
 	case *builder.MultiError:
 		for _, err := range err.Errs {
 			printCompilerError(logln, err)
@@ -982,11 +952,31 @@ func main() {
 			usage()
 			os.Exit(1)
 		}
-		err = runGoList(config, *flagJSON, *flagDeps, flag.Args())
+		var extraArgs []string
+		if *flagJSON {
+			extraArgs = append(extraArgs, "-json")
+		}
+		if *flagDeps {
+			extraArgs = append(extraArgs, "-deps")
+		}
+		cmd, err := loader.List(config, extraArgs, flag.Args())
 		if err != nil {
 			fmt.Fprintln(os.Stderr, "failed to run `go list`:", err)
 			os.Exit(1)
 		}
+		cmd.Stdout = os.Stdout
+		cmd.Stderr = os.Stderr
+		err = cmd.Run()
+		if err != nil {
+			if exitErr, ok := err.(*exec.ExitError); ok {
+				if status, ok := exitErr.Sys().(syscall.WaitStatus); ok {
+					os.Exit(status.ExitStatus())
+				}
+				os.Exit(1)
+			}
+			fmt.Fprintln(os.Stderr, "failed to run `go list`:", err)
+			os.Exit(1)
+		}
 	case "clean":
 		// remove cache directory
 		err := os.RemoveAll(goenv.Get("GOCACHE"))