Migrate auto-configuration registration

- Move auto-configs from spring.factories to imports file.
- Relates #523

Author: jvalkeal

spring-shell-autoconfigure/src/main/resources/META-INF/spring.factories

@@ -0,0 +1,16 @@
+org.springframework.shell.boot.ExitCodeAutoConfiguration
+org.springframework.shell.boot.ShellContextAutoConfiguration
+org.springframework.shell.boot.SpringShellAutoConfiguration
+org.springframework.shell.boot.ShellRunnerAutoConfiguration
+org.springframework.shell.boot.ApplicationRunnerAutoConfiguration
+org.springframework.shell.boot.CommandCatalogAutoConfiguration
+org.springframework.shell.boot.LineReaderAutoConfiguration
+org.springframework.shell.boot.CompleterAutoConfiguration
+org.springframework.shell.boot.UserConfigAutoConfiguration
+org.springframework.shell.boot.JLineAutoConfiguration
+org.springframework.shell.boot.JLineShellAutoConfiguration
+org.springframework.shell.boot.ParameterResolverAutoConfiguration
+org.springframework.shell.boot.StandardAPIAutoConfiguration
+org.springframework.shell.boot.ThemingAutoConfiguration
+org.springframework.shell.boot.StandardCommandsAutoConfiguration
+org.springframework.shell.boot.ComponentFlowAutoConfiguration