Refactor com.vaadin.flow.spring.VaadinServletContextInitializer

[mshabarov]

Describe your motivation

Code Analysis highlights:

• The module has at least 3 different responsibilities amongst its 48 functions.

• This file has 2 bumpy roads.
  VaadinServletContextInitializer.CustomResourceLoader.collectResources(bumps = 6)
  VaadinServletContextInitializer(bumps = 2)
  A Bumpy Road is a function that contains multiple chunks of nested conditional logic inside the same function. The deeper the nesting and the more bumps, the lower the code health.

• The file contains 1 functions with deeply nested conditional logic.
  VaadinServletContextInitializer.CustomResourceLoader.collectResources(Nesting depth = 5 conditionals)

• 4 methods have high complexity:
  VaadinServletContextInitializer.CustomResourceLoader.collectResources(cc = 19)
  VaadinServletContextInitializer.DevModeServletContextListener.failFastContextInitialized(cc = 18)
  VaadinServletContextInitializer.RouteServletContextListener.failFastContextInitialized(cc = 14)
  VaadinServletContextInitializer(cc = 9)### Describe the solution you'd like

• One method has complex conditionals.
  VaadinServletContextInitializer.DevModeServletContextListener.failFastContextInitialized:579(2 complex conditional expressions)

• You have 3 functions that exceed the threshold.
  VaadinServletContextInitializer.DevModeServletContextListener.failFastContextInitialized(LoC = 77 lines)
  VaadinServletContextInitializer.RouteServletContextListener.failFastContextInitialized(LoC = 75 lines)
  VaadinServletContextInitializer.CustomResourceLoader.collectResources(LoC = 73 lines)

Additional context

See CodeScene analysis.