FetchContent: Relative FETCHCONTENT_SOURCE_DIR_depName fails existence check

Originally reported in the forums here. If the user provides a relative path for a FETCHCONTENT_SOURCE_DIR_<depName> override, we can't reliably check for its existence. This is because we don't know what directory that path was expected to be relative to. The implementation will simply pass it back to the caller in the <depName>_SOURCE_DIR variable. That means the caller has to handle the path being relative.

Some projects rely on the above behavior, so to preserve backward compatibility, the best we can do is warn about this and by-pass the existence check.