Restructure embedded web server packages

Rework `org.springframework.boot.context.embedded` to relocate classes
to `org.springframework.boot.web`. Packages are now organized around
the following areas:

Packages for shared concerns, for example the `WebServer` interface
to start/stop a server and the common configuration elements:
- org.springframework.boot.web.context
- org.springframework.boot.web.server

Servlet specific packages:
- org.springframework.boot.web.servlet.server
- org.springframework.boot.web.servlet.context
- org.springframework.boot.web.servlet.filter

Reactive specific packages:
- org.springframework.boot.web.reactive.context
- org.springframework.boot.web.reactive.server

Embedded server implementations (both reactive and servlet):
- org.springframework.boot.web.embedded

In addition:

- Rename `EmbeddedServletContainerFactory` to `ServletWebServerFactory`
  to align with the `ReactiveWebServerFactory`.
- Rename `EmbeddedWebApplicationContext` to
  `ServletWebServerApplicationContext` and
- Rename `EmbeddedReactiveWebApplicationContext` to
  `ReactiveWebServerApplicationContext`.
- Add checkstyle rules to restrict imports.
- Fixup all affected code to use the correct imports and local names.

Fixes gh-8532

spring-boot-docs/src/main/java/org/springframework/boot/context/embedded/TomcatLegacyCookieProcessorExample.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2016 the original author or authors.
+ * Copyright 2012-2017 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -19,8 +19,10 @@ package org.springframework.boot.context.embedded;
 import org.apache.catalina.Context;
 import org.apache.tomcat.util.http.LegacyCookieProcessor;
 
-import org.springframework.boot.context.embedded.tomcat.TomcatContextCustomizer;
-import org.springframework.boot.context.embedded.tomcat.TomcatEmbeddedServletContainerFactory;
+import org.springframework.boot.web.embedded.tomcat.TomcatContextCustomizer;
+import org.springframework.boot.web.embedded.tomcat.TomcatServletWebServerFactory;
+import org.springframework.boot.web.servlet.server.ConfigurableServletWebServerFactory;
+import org.springframework.boot.web.servlet.server.ServletWebServerFactoryCustomizer;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 
@@ -33,20 +35,20 @@ public class TomcatLegacyCookieProcessorExample {
 
 	/**
 	 * Configuration class that declares the required
-	 * {@link EmbeddedServletContainerCustomizer}.
+	 * {@link ServletWebServerFactoryCustomizer}.
 	 */
 	@Configuration
 	static class LegacyCookieProcessorConfiguration {
 
 		// tag::customizer[]
 		@Bean
-		public EmbeddedServletContainerCustomizer cookieProcessorCustomizer() {
-			return new EmbeddedServletContainerCustomizer() {
+		public ServletWebServerFactoryCustomizer cookieProcessorCustomizer() {
+			return new ServletWebServerFactoryCustomizer() {
 
 				@Override
-				public void customize(ConfigurableEmbeddedServletContainer container) {
-					if (container instanceof TomcatEmbeddedServletContainerFactory) {
-						((TomcatEmbeddedServletContainerFactory) container)
+				public void customize(ConfigurableServletWebServerFactory serverFactory) {
+					if (serverFactory instanceof TomcatServletWebServerFactory) {
+						((TomcatServletWebServerFactory) serverFactory)
 								.addContextCustomizers(new TomcatContextCustomizer() {
 
 							@Override