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-samples/spring-boot-sample-tomcat/src/test/java/sample/tomcat/NonAutoConfigurationSampleTomcatApplicationTests.java

@@ -25,7 +25,7 @@ import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.boot.SpringApplication;
 import org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration;
 import org.springframework.boot.autoconfigure.web.DispatcherServletAutoConfiguration;
-import org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration;
+import org.springframework.boot.autoconfigure.web.ServletWebServerFactoryAutoConfiguration;
 import org.springframework.boot.autoconfigure.web.HttpMessageConvertersAutoConfiguration;
 import org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration;
 import org.springframework.boot.test.context.SpringBootTest;
@@ -55,7 +55,7 @@ public class NonAutoConfigurationSampleTomcatApplicationTests {
 	private TestRestTemplate restTemplate;
 
 	@Configuration
-	@Import({ EmbeddedServletContainerAutoConfiguration.class,
+	@Import({ ServletWebServerFactoryAutoConfiguration.class,
 			DispatcherServletAutoConfiguration.class, WebMvcAutoConfiguration.class,
 			HttpMessageConvertersAutoConfiguration.class,
 			PropertyPlaceholderAutoConfiguration.class })

spring-boot-samples/spring-boot-sample-tomcat/src/test/java/sample/tomcat/SampleTomcatApplicationTests.java

@@ -26,11 +26,11 @@ import org.junit.Test;
 import org.junit.runner.RunWith;
 
 import org.springframework.beans.factory.annotation.Autowired;
-import org.springframework.boot.context.embedded.EmbeddedWebApplicationContext;
-import org.springframework.boot.context.embedded.tomcat.TomcatEmbeddedServletContainer;
 import org.springframework.boot.test.context.SpringBootTest;
 import org.springframework.boot.test.context.SpringBootTest.WebEnvironment;
 import org.springframework.boot.test.web.client.TestRestTemplate;
+import org.springframework.boot.web.embedded.tomcat.TomcatWebServer;
+import org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext;
 import org.springframework.context.ApplicationContext;
 import org.springframework.http.HttpEntity;
 import org.springframework.http.HttpHeaders;
@@ -88,9 +88,9 @@ public class SampleTomcatApplicationTests {
 
 	@Test
 	public void testTimeout() throws Exception {
-		EmbeddedWebApplicationContext context = (EmbeddedWebApplicationContext) this.applicationContext;
-		TomcatEmbeddedServletContainer embeddedServletContainer = (TomcatEmbeddedServletContainer) context
-				.getEmbeddedWebServer();
+		ServletWebServerApplicationContext context = (ServletWebServerApplicationContext) this.applicationContext;
+		TomcatWebServer embeddedServletContainer = (TomcatWebServer) context
+				.getWebServer();
 		ProtocolHandler protocolHandler = embeddedServletContainer.getTomcat()
 				.getConnector().getProtocolHandler();
 		int timeout = ((AbstractProtocol<?>) protocolHandler).getConnectionTimeout();