cache: fix build failure on Windows

cache.go

@@ -6,11 +6,9 @@ import (
 	"encoding/json"
 	"os"
 	"path/filepath"
-	"runtime"
 	"sort"
 	"strings"
 	"sync"
-	"syscall"
 	"time"
 )
 
@@ -26,16 +24,6 @@ var projectTombstonesUncommon = [...]string{
 	".hg",
 }
 
-func fileExists(name string) bool {
-	if runtime.GOOS != "windows" {
-		var stat syscall.Stat_t
-		return syscall.Lstat(name, &stat) == nil
-	} else {
-		_, err := os.Lstat(name)
-		return err == nil
-	}
-}
-
 func projectDir(dir string) string {
 	dir = filepath.Clean(dir)
 	orig := dir

file_exist_portable.go

@@ -0,0 +1,11 @@
+//go:build windows || plan9
+// +build windows plan9
+
+package imports
+
+import "os"
+
+func fileExists(name string) bool {
+	_, err := os.Lstat(name)
+	return err == nil
+}

file_exist_unix.go

@@ -0,0 +1,11 @@
+//go:build !windows && !plan9
+// +build !windows,!plan9
+
+package imports
+
+import "syscall"
+
+func fileExists(name string) bool {
+	var stat syscall.Stat_t
+	return syscall.Lstat(name, &stat) == nil
+}