can not delete cache file

```
if (toRemove.contains(cacheObject.id)) return;

toRemove.add(cacheObject.id!);
if (_memCache.containsKey(cacheObject.key)) {
_memCache.remove(cacheObject.key);
}
if (_futureCache.containsKey(cacheObject.key)) {
await _futureCache.remove(cacheObject.key);
}
final file = io.File(cacheObject.relativePath);

if (file.existsSync()) {
try {
await file.delete();
// ignore: unused_catch_clause
} on PathNotFoundException catch (e) {
// File has already been deleted. Do nothing #184
}
}
```
cacheObject.relativePath is not real abs file path ,delete file can not work
`class CustomCacheManager {
static const key = 'tsl';
static final fileSystem = ExternalStorageFileSystem(key);
static CacheManager instance = CacheManager(
Config(key,
stalePeriod: const Duration(days: 30),
maxNrOfCacheObjects: 999999999,
fileSystem: fileSystem),
);
}`
`
class ExternalStorageFileSystem implements FileSystem {
final Future _fileDir;
final String _cacheKey;

ExternalStorageFileSystem(this._cacheKey)
: _fileDir = createDirectory(_cacheKey);

static Future createDirectory(String key) async {
final baseDir = await getExternalStorageDirectory();
final path = p.join(baseDir!.path, key);

const fs = LocalFileSystem();
final directory = fs.directory(path);
await directory.create(recursive: true);
return directory;
}

@override
Future createFile(String name) async {
final directory = await _fileDir;
if (!(await directory.exists())) {
await createDirectory(_cacheKey);
}
return directory.childFile(name);
}

void emptyCache() async {
final directory = await _fileDir;
if (await directory.exists()) {
await directory.delete(recursive: true);
}
}
}
`

首先跟踪 issue 中所示的 cacheObject.relativePath 删除路径和 ExternalStorageFileSystem.createFile 实现。检查文件路径是如何生成和使用的，然后验证删除缓存对象时会删除外部存储中的相应文件，同时不会破坏现有的缺失文件处理逻辑。当此文件系统创建的缓存文件可以成功删除时，即视为完成。

由索引模型根据 Issue 内容生成。