Prior to this commit, defining a custom `DispatcherServlet` and/or a
`ServletRegistrationBean` with the default name would turn off
completely the `DispatcherServletAutoConfiguration`.
This commit splits this auto-configuration in two parts:
- First, a `DispatcherServlet` is automatically registered if no
instance is already defined with the default name.
- Then, a `ServletRegistrationBean` is registered is registered if a
`DispatcherServlet` instance exists with the default name *and* no
`ServletRegistrationBean` exists with the default name
This allows developers to register manually a `ServletRegistrationBean`
or a `DispatcherServlet` without having to redefine the whole
auto-configuration.
Fixes gh-4893
Closes gh-6108
091664078f