Move internal packages to an internal folder

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

internal/worker/worker.go

@@ -0,0 +1,46 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package worker // import "miniflux.app/v2/internal/worker"
+
+import (
+	"time"
+
+	"miniflux.app/v2/internal/config"
+	"miniflux.app/v2/internal/logger"
+	"miniflux.app/v2/internal/metric"
+	"miniflux.app/v2/internal/model"
+	feedHandler "miniflux.app/v2/internal/reader/handler"
+	"miniflux.app/v2/internal/storage"
+)
+
+// Worker refreshes a feed in the background.
+type Worker struct {
+	id    int
+	store *storage.Storage
+}
+
+// Run wait for a job and refresh the given feed.
+func (w *Worker) Run(c chan model.Job) {
+	logger.Debug("[Worker] #%d started", w.id)
+
+	for {
+		job := <-c
+		logger.Debug("[Worker #%d] Received feed #%d for user #%d", w.id, job.FeedID, job.UserID)
+
+		startTime := time.Now()
+		refreshErr := feedHandler.RefreshFeed(w.store, job.UserID, job.FeedID, false)
+
+		if config.Opts.HasMetricsCollector() {
+			status := "success"
+			if refreshErr != nil {
+				status = "error"
+			}
+			metric.BackgroundFeedRefreshDuration.WithLabelValues(status).Observe(time.Since(startTime).Seconds())
+		}
+
+		if refreshErr != nil {
+			logger.Error("[Worker] Refreshing the feed #%d returned this error: %v", job.FeedID, refreshErr)
+		}
+	}
+}