Move internal packages to an internal folder

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

oauth2/manager.go

@@ -1,46 +0,0 @@
-// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
-// SPDX-License-Identifier: Apache-2.0
-
-package oauth2 // import "miniflux.app/v2/oauth2"
-
-import (
-	"context"
-	"errors"
-
-	"miniflux.app/v2/logger"
-)
-
-// Manager handles OAuth2 providers.
-type Manager struct {
-	providers map[string]Provider
-}
-
-// FindProvider returns the given provider.
-func (m *Manager) FindProvider(name string) (Provider, error) {
-	if provider, found := m.providers[name]; found {
-		return provider, nil
-	}
-
-	return nil, errors.New("oauth2 provider not found")
-}
-
-// AddProvider add a new OAuth2 provider.
-func (m *Manager) AddProvider(name string, provider Provider) {
-	m.providers[name] = provider
-}
-
-// NewManager returns a new Manager.
-func NewManager(ctx context.Context, clientID, clientSecret, redirectURL, oidcDiscoveryEndpoint string) *Manager {
-	m := &Manager{providers: make(map[string]Provider)}
-	m.AddProvider("google", newGoogleProvider(clientID, clientSecret, redirectURL))
-
-	if oidcDiscoveryEndpoint != "" {
-		if genericOidcProvider, err := newOidcProvider(ctx, clientID, clientSecret, redirectURL, oidcDiscoveryEndpoint); err != nil {
-			logger.Error("[OAuth2] failed to initialize OIDC provider: %v", err)
-		} else {
-			m.AddProvider("oidc", genericOidcProvider)
-		}
-	}
-
-	return m
-}