cmTarget: add support for C++ module fileset types

C++ modules have two variants which are of importance to CMake:

  - `CXX_MODULES`: interface modules (those using `export module M;`,
    `export module M:part;`, or `module M:internal_part;`)
  - `CXX_MODULE_HEADER_UNITS`: importable header units

Creating C++ modules or partitions are *not* supported in any other
source listing. This is because the source files must be installed (so
their scope matters), but not part of usage requirements (what it means
for a module source to be injected into a consumer is not clear at this
moment). Due to the way `FILE_SET` works with scopes, they are a perfect
fit as long as `INTERFACE` is not allowed (which it is not).

1 files changed, 0 insertions, 5 deletions

diff --git a/Help/dev/experimental.rst b/Help/dev/experimental.rst
index 2c2eb21..14e6075 100644
--- a/Help/dev/experimental.rst
+++ b/Help/dev/experimental.rst
@@ -57,11 +57,6 @@ dependencies to the file specified by the ``<DYNDEP_FILE>`` placeholder. The
 ``CMAKE_EXPERIMENTAL_CXX_SCANDEP_DEPFILE_FORMAT`` file may be set to ``msvc``
 for scandep rules which use ``msvc``-style dependency reporting.
 
-For tools which need to know the file set the source belongs to, the
-``CMAKE_EXPERIMENTAL_CXX_MODULE_SOURCE_TYPE_FLAG_<FILE_SET_TYPE>`` flag may
-be provided so that different source types can be distinguished prior to
-scanning.
-
 The module dependencies should be written in the format described
 by the `P1689r4`_ paper.