exports: support `CXX_MODULES_DIRECTORY`

This directory will be used to store build-discovered information about
targets such as the modules provided by the files in the relevant
`FILE_SET` types.

A directory is used because basing the name on a `<FILE_NAME>-*.cmake`
pattern makes it end up being globbed in the configuration-dependent
information mechanism. Since old modules and targets may be around,
unconditionally including them may refer to targets that do not actually
exist.