Make litho/sections a separate plugin

Summary:
Breaking Litho/Sections off. Doesn't really make sense to have
two deps for this especially since they're pretty tightly coupled.

This makes our core already quite slim.

Reviewed By: jknoxville

Differential Revision: D17420118

fbshipit-source-id: 9a03911f4af6410745b9aefd0e6a75bdf106660f

android/sample/build.gradle

@@ -69,5 +69,6 @@ dependencies {
     debugImplementation project(':android')
     debugImplementation project(':fresco-plugin')
     debugImplementation project(':network-plugin')
+    debugImplementation project(':litho-plugin')
     releaseImplementation project(':noop')
 }