Fixes: junit-team#3630

…ources-scanning

As a follow up for junit-team#3630 and junit-team#3705 this adds a
`addResourceContainerSelectorResolver()`
method to `EngineDiscoveryRequestResolver.Builder` analogous to
`addClassContainerSelectorResolver()`.

Points of note:

 * As classpath resources can be selected from packages, the package
   filter should also be applied. To make this possible the base path of
   a resource is rewritten to a package name prior to being filtered.

 * The `ClasspathResourceSelector` now has a `getClasspathResource`
   method. This method will lazily try to load the resource if not was
   not already provided when discovering resources in a container.

 * `selectClasspathResource(Resource)` was added to short circuit the
    need to resolve resources twice. And to make it possible to use
    this method as part of the public API,
    `ReflectionSupport.tryToLoadResource` was also added.