refactor: move act under internal

The runner is an application, not a library. `act/model` and
`act/exprparser` were the last packages anything outside this repository
consumed and they now live in actionslib, so nothing needs the rest of
`act` to be importable, and keeping it importable invites the coupling
that was just removed.

Import paths only, the files are unchanged.

Assisted-by: Codet:GPT-5.1-Codex

internal/act/artifactcache/doc.go

@@ -0,0 +1,12 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// Copyright 2023 The nektos/act Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+// Package artifactcache provides a cache handler for the runner.
+//
+// Inspired by https://github.com/sp-ricard-valverde/github-act-cache-server
+//
+// TODO: Authorization
+// TODO: Restrictions for accessing a cache, see https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows#restrictions-for-accessing-a-cache
+// TODO: Force deleting cache entries, see https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows#force-deleting-cache-entries
+package artifactcache