os: remove dependency on strings package

Historically the os package has not imported the strings package.
That was enforced by go/build.TestDependencies, but that test
was accidentally broken (#43249). A dependency of os on strings
was accidentally added by CL 266364; remove it.

For #42026
For #43249

Change-Id: If932308f30561fdcc5c608d7563e849c0d2870d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/279072
Trust: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Tobias Klauser <[email protected]>

Author: ianlancetaylor

src/os/file_plan9.go

@@ -336,16 +336,6 @@ func hasPrefix(s, prefix string) bool {
 	return len(s) >= len(prefix) && s[0:len(prefix)] == prefix
 }
 
-// LastIndexByte from the strings package.
-func lastIndex(s string, sep byte) int {
-	for i := len(s) - 1; i >= 0; i-- {
-		if s[i] == sep {
-			return i
-		}
-	}
-	return -1
-}
-
 func rename(oldname, newname string) error {
 	dirname := oldname[:lastIndex(oldname, '/')+1]
 	if hasPrefix(newname, dirname) {

src/os/tempfile.go

@@ -4,10 +4,7 @@
 
 package os
 
-import (
-	"errors"
-	"strings"
-)
+import "errors"
 
 // fastrand provided by runtime.
 // We generate random temporary file names so that there's a good
@@ -62,7 +59,7 @@ func prefixAndSuffix(pattern string) (prefix, suffix string, err error) {
 			return "", "", errPatternHasSeparator
 		}
 	}
-	if pos := strings.LastIndex(pattern, "*"); pos != -1 {
+	if pos := lastIndex(pattern, '*'); pos != -1 {
 		prefix, suffix = pattern[:pos], pattern[pos+1:]
 	} else {
 		prefix = pattern
@@ -116,3 +113,13 @@ func joinPath(dir, name string) string {
 	}
 	return dir + string(PathSeparator) + name
 }
+
+// LastIndexByte from the strings package.
+func lastIndex(s string, sep byte) int {
+	for i := len(s) - 1; i >= 0; i-- {
+		if s[i] == sep {
+			return i
+		}
+	}
+	return -1
+}