Refactor to new framework (#98)

- Adjust directory structure
```text
├── internal
│   ├── app
│   │   ├── artifactcache
│   │   ├── cmd
│   │   ├── poll
│   │   └── run
│   └── pkg
│       ├── client
│       ├── config
│       ├── envcheck
│       ├── labels
│       ├── report
│       └── ver
└── main.go
```
- New pkg `labels` to parse label
- New pkg `report` to report logs to Gitea
- Remove pkg `engine`, use `envcheck` to check if docker running.
- Rewrite `runtime` to `run`
- Rewrite `poller` to `poll`
- Simplify some code and remove what's useless.

Reviewed-on: https://gitea.com/gitea/act_runner/pulls/98
Reviewed-by: Lunny Xiao <[email protected]>
Co-authored-by: Jason Song <[email protected]>
Co-committed-by: Jason Song <[email protected]>

internal/pkg/envcheck/doc.go

@@ -0,0 +1,5 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+// Package envcheck provides a simple way to check if the environment is ready to run jobs.
+package envcheck