The previous 'openModuleFiles' interface in SerializedModuleLoaderBase
still assumed that swiftmodule files and swiftdoc files would be found
next to each other, but that's not true anymore with
swiftinterfaces-built-to-modules. Give up on this assumption (and on
the minor optimization of passing down a scratch buffer) and split out
the interface into the customization point
'findModuleFilesInDirectory' and the implementation 'openModuleFiles'.
The latter now takes two full paths: one for the swiftmodule, one for
the swiftdoc.
