Move internal packages to an internal folder

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

ui/static_stylesheet.go

@@ -1,29 +0,0 @@
-// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
-// SPDX-License-Identifier: Apache-2.0
-
-package ui // import "miniflux.app/v2/ui"
-
-import (
-	"net/http"
-	"time"
-
-	"miniflux.app/v2/http/request"
-	"miniflux.app/v2/http/response"
-	"miniflux.app/v2/http/response/html"
-	"miniflux.app/v2/ui/static"
-)
-
-func (h *handler) showStylesheet(w http.ResponseWriter, r *http.Request) {
-	filename := request.RouteStringParam(r, "name")
-	etag, found := static.StylesheetBundleChecksums[filename]
-	if !found {
-		html.NotFound(w, r)
-		return
-	}
-
-	response.New(w, r).WithCaching(etag, 48*time.Hour, func(b *response.Builder) {
-		b.WithHeader("Content-Type", "text/css; charset=utf-8")
-		b.WithBody(static.StylesheetBundles[filename])
-		b.Write()
-	})
-}