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, 18 insertions, 0 deletions

diff --git a/Help/prop_tgt/CXX_MODULE_SET.rst b/Help/prop_tgt/CXX_MODULE_SET.rst
new file mode 100644
index 0000000..ae9000e
--- /dev/null
+++ b/Help/prop_tgt/CXX_MODULE_SET.rst
@@ -0,0 +1,18 @@
+CXX_MODULE_SET
+--------------
+
+.. note ::
+
+  Experimental. Gated by ``CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API``
+
+Semicolon-separated list of files in the target's default C++ module set,
+(i.e. the file set with name and type ``CXX_MODULES``). If any of the paths
+are relative, they are computed relative to the target's source directory. The
+property supports
+:manual:`generator expressions <cmake-generator-expressions(7)>`.
+
+This property is normally only set by :command:`target_sources(FILE_SET)`
+rather than being manipulated directly.
+
+See :prop_tgt:`CXX_MODULE_SET_<NAME>` for the list of files in other C++
+module sets.