Move internal packages to an internal folder

For reference: https://go.dev/doc/go1.4#internalpackages

internal/worker/pool.go

@@ -0,0 +1,35 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package worker // import "miniflux.app/v2/internal/worker"
+
+import (
+	"miniflux.app/v2/internal/model"
+	"miniflux.app/v2/internal/storage"
+)
+
+// Pool handles a pool of workers.
+type Pool struct {
+	queue chan model.Job
+}
+
+// Push send a list of jobs to the queue.
+func (p *Pool) Push(jobs model.JobList) {
+	for _, job := range jobs {
+		p.queue <- job
+	}
+}
+
+// NewPool creates a pool of background workers.
+func NewPool(store *storage.Storage, nbWorkers int) *Pool {
+	workerPool := &Pool{
+		queue: make(chan model.Job),
+	}
+
+	for i := 0; i < nbWorkers; i++ {
+		worker := &Worker{id: i, store: store}
+		go worker.Run(workerPool.queue)
+	}
+
+	return workerPool
+}