cmNinjaTargetGenerator: do not order-depend on C++ module sources

C++ module sources should not be included by any other TUs, so their
presence cannot matter for order-only dependencies of the entire target.
Exclude them.

Update CMP0154 to take this into consideration and add tests to the
`CXXModules` suite (which already deals with module support detection).

Help/policy/CMP0154.rst

@@ -22,6 +22,12 @@ type ``HEADERS``.  With this information, :ref:`Ninja Generators` may omit
 the above-mentioned conservative dependencies and produce more efficient
 build graphs.
 
+Additionally, if the custom command's output is a member of a file set of type
+``CXX_MODULES``, it will additionally not be required to exist before
+compiling other sources in the same target.  Since these files should not be
+included at compile time directly, they may not be implicitly required to
+exist for other compilation rules.
+
 This policy provides compatibility for projects using file sets in targets
 with generated header files that have not been updated.  Such projects
 should be updated to express generated public headers in a file set.