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/test/java/org/springframework/boot/context/embedded/TomcatLegacyCookieProcessorExampleTests.java

@@ -22,8 +22,10 @@ import org.junit.Test;
 
 import org.springframework.boot.SpringApplication;
 import org.springframework.boot.context.embedded.TomcatLegacyCookieProcessorExample.LegacyCookieProcessorConfiguration;
-import org.springframework.boot.context.embedded.tomcat.TomcatEmbeddedServletContainer;
-import org.springframework.boot.context.embedded.tomcat.TomcatEmbeddedServletContainerFactory;
+import org.springframework.boot.web.embedded.tomcat.TomcatServletWebServerFactory;
+import org.springframework.boot.web.embedded.tomcat.TomcatWebServer;
+import org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext;
+import org.springframework.boot.web.servlet.server.ServletWebServerFactoryCustomizerBeanPostProcessor;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 
@@ -38,10 +40,10 @@ public class TomcatLegacyCookieProcessorExampleTests {
 
 	@Test
 	public void cookieProcessorIsCustomized() {
-		EmbeddedWebApplicationContext applicationContext = (EmbeddedWebApplicationContext) new SpringApplication(
+		ServletWebServerApplicationContext applicationContext = (ServletWebServerApplicationContext) new SpringApplication(
 				TestConfiguration.class, LegacyCookieProcessorConfiguration.class).run();
-		Context context = (Context) ((TomcatEmbeddedServletContainer) applicationContext
-				.getEmbeddedWebServer()).getTomcat().getHost().findChildren()[0];
+		Context context = (Context) ((TomcatWebServer) applicationContext
+				.getWebServer()).getTomcat().getHost().findChildren()[0];
 		assertThat(context.getCookieProcessor())
 				.isInstanceOf(LegacyCookieProcessor.class);
 	}
@@ -50,13 +52,13 @@ public class TomcatLegacyCookieProcessorExampleTests {
 	static class TestConfiguration {
 
 		@Bean
-		public TomcatEmbeddedServletContainerFactory tomcatFactory() {
-			return new TomcatEmbeddedServletContainerFactory(0);
+		public TomcatServletWebServerFactory tomcatFactory() {
+			return new TomcatServletWebServerFactory(0);
 		}
 
 		@Bean
-		public EmbeddedServletContainerCustomizerBeanPostProcessor postProcessor() {
-			return new EmbeddedServletContainerCustomizerBeanPostProcessor();
+		public ServletWebServerFactoryCustomizerBeanPostProcessor postProcessor() {
+			return new ServletWebServerFactoryCustomizerBeanPostProcessor();
 		}
 
 	}