Update `StandardConfigDataLocationResolver` to no longer check if
directories exist for classpath resources. Unfortunately checking for
the parent directory of a `ClassPathResource` isn't always possible
without resorting something similar to the
`PathMatchingResourcePatternResolver` which would add a lot of
complexity to the resolver.

In order to ensure that non-optional locations are always resolved,
the `ConfigDataEnvironment` now checks that all imported locations
have been loaded.

Closes gh-24143