Refine Mustache support

Refine Mustache support to provide a cleaner separation between the
reactive and servlet implementations. The views have now moved to the
`spring-boot` project and the auto-configuration has been split into
two distinct `@Imports` to save needing full package declarations.

See gh-8941

spring-boot-parent/src/checkstyle/import-control.xml

@@ -50,6 +50,9 @@
 				<allow pkg="org.springframework.boot.web.servlet" />
 				<allow pkg="org.springframework.boot.web.server" />
 			</subpackage>
+			<subpackage name="view">
+				<allow pkg="org.springframework.web.servlet" />
+			</subpackage>
 		</subpackage>
 
 		<!-- Reactive -->
@@ -63,6 +66,11 @@
 				<allow pkg="org.springframework.boot.web.server" />
 				<disallow pkg="org.springframework.context" />
 			</subpackage>
+			<subpackage name="result">
+				<subpackage name="view">
+					<allow pkg="org.springframework.boot.web.reactive.result.view" />
+				</subpackage>
+			</subpackage>
 		</subpackage>
 
 		<!-- Embedded Servers -->