Move ServletContextResourceFilePathResolver into org.springframework.boot.web.servlet.context

[wilkinsona]

ServletContextResourceFilePathResolver is in org.springframework.boot.web.context at the moment where it's the only servlet-specific class in the package. I think it should probably move into org.springframework.boot.web.servlet.context instead. That would also provide some symmetry with its reactive equivalent, FilteredReactiveWebContextResourceFilePathResolver, which is in org.springframework.boot.web.reactive.context.