Move internal packages to an internal folder

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

internal/api/subscription.go

@@ -0,0 +1,48 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package api // import "miniflux.app/v2/internal/api"
+
+import (
+	json_parser "encoding/json"
+	"net/http"
+
+	"miniflux.app/v2/internal/http/response/json"
+	"miniflux.app/v2/internal/model"
+	"miniflux.app/v2/internal/reader/subscription"
+	"miniflux.app/v2/internal/validator"
+)
+
+func (h *handler) discoverSubscriptions(w http.ResponseWriter, r *http.Request) {
+	var subscriptionDiscoveryRequest model.SubscriptionDiscoveryRequest
+	if err := json_parser.NewDecoder(r.Body).Decode(&subscriptionDiscoveryRequest); err != nil {
+		json.BadRequest(w, r, err)
+		return
+	}
+
+	if validationErr := validator.ValidateSubscriptionDiscovery(&subscriptionDiscoveryRequest); validationErr != nil {
+		json.BadRequest(w, r, validationErr.Error())
+		return
+	}
+
+	subscriptions, finderErr := subscription.FindSubscriptions(
+		subscriptionDiscoveryRequest.URL,
+		subscriptionDiscoveryRequest.UserAgent,
+		subscriptionDiscoveryRequest.Cookie,
+		subscriptionDiscoveryRequest.Username,
+		subscriptionDiscoveryRequest.Password,
+		subscriptionDiscoveryRequest.FetchViaProxy,
+		subscriptionDiscoveryRequest.AllowSelfSignedCertificates,
+	)
+	if finderErr != nil {
+		json.ServerError(w, r, finderErr)
+		return
+	}
+
+	if subscriptions == nil {
+		json.NotFound(w, r)
+		return
+	}
+
+	json.OK(w, r, subscriptions)
+}