refactor: retry file remove/rename when a file is busy and clean up os detection (#38588)

Rename functions "util.Remove" (remove.go) to "util.RemoveWithRetry"
(file_retry.go) and add comments to clarify their behaviors, also add
tests.

Refactor callers: when no concurrent access (cmd cli, migration, app
init, test), use "os.Xxx" directly.

More details are in `modules/util/file_retry.go`

By the way, clean up OS (windows) detection, make FileURLToPath test
always run

models/repo/upload.go

@@ -127,7 +127,7 @@ func DeleteUploads(ctx context.Context, uploads ...*Upload) (err error) {
 
 	for _, upload := range uploads {
 		localPath := upload.LocalPath()
-		if err := util.Remove(localPath); err != nil {
+		if err := util.RemoveWithRetry(localPath); err != nil {
 			// just continue, don't fail the whole operation if a file is missing (removed by others)
 			log.Error("unable to remove upload file %s: %v", localPath, err)
 		}

models/user/user.go

@@ -994,6 +994,7 @@ func GetInactiveUsers(ctx context.Context, olderThan time.Duration) ([]*User, er
 }
 
 // UserPath returns the path absolute path of user repositories.
+// FIXME: it should be in "git/gitrepo" package
 func UserPath(userName string) string { //revive:disable-line:exported
 	return filepath.Join(setting.RepoRootPath, filepath.Clean(strings.ToLower(userName)))
 }