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-web-method-security/src/test/java/sample/security/method/SampleMethodSecurityApplicationTests.java

@@ -24,10 +24,10 @@ import org.junit.Test;
 import org.junit.runner.RunWith;
 
 import org.springframework.beans.factory.annotation.Autowired;
-import org.springframework.boot.context.embedded.LocalServerPort;
 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.server.LocalServerPort;
 import org.springframework.http.HttpEntity;
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpMethod;