os,text,unicode: renamings

This is Go 1 package renaming CL #4.
This one merely moves the source; the import strings will be
changed after the next weekly release.
This one moves pieces into os, text, and unicode.

exec -> os/exec
scanner -> text/scanner
tabwriter -> text/tabwriter
template -> text/template
template/parse -> text/template/parse
utf16 -> unicode/utf16
utf8 -> unicode/utf8

This should be the last of the source-rearranging CLs.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5331066

src/pkg/os/exec/lp_unix.go

@@ -0,0 +1,55 @@
+// Copyright 2010 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build darwin freebsd linux openbsd
+
+package exec
+
+import (
+	"errors"
+	"os"
+	"strings"
+)
+
+// ErrNotFound is the error resulting if a path search failed to find an executable file.
+var ErrNotFound = errors.New("executable file not found in $PATH")
+
+func findExecutable(file string) error {
+	d, err := os.Stat(file)
+	if err != nil {
+		return err
+	}
+	if d.IsRegular() && d.Permission()&0111 != 0 {
+		return nil
+	}
+	return os.EPERM
+}
+
+// LookPath searches for an executable binary named file
+// in the directories named by the PATH environment variable.
+// If file contains a slash, it is tried directly and the PATH is not consulted.
+func LookPath(file string) (string, error) {
+	// NOTE(rsc): I wish we could use the Plan 9 behavior here
+	// (only bypass the path if file begins with / or ./ or ../)
+	// but that would not match all the Unix shells.
+
+	if strings.Contains(file, "/") {
+		err := findExecutable(file)
+		if err == nil {
+			return file, nil
+		}
+		return "", &Error{file, err}
+	}
+	pathenv := os.Getenv("PATH")
+	for _, dir := range strings.Split(pathenv, ":") {
+		if dir == "" {
+			// Unix shell semantics: path element "" means "."
+			dir = "."
+		}
+		if err := findExecutable(dir + "/" + file); err == nil {
+			return dir + "/" + file, nil
+		}
+	}
+	return "", &Error{file, ErrNotFound}
+}