Autogen: Add AUTOMOC support for C++ module units

Process C++ module interface and partition units that are members of a
FILE_SET of type CXX_MODULES.  moc is run on such units and its
generated output is compiled as a module implementation unit of the same
module, as its own scanned translation unit rather than folded into
mocs_compilation.cpp.  Macro-less units get an empty placeholder; module
units and their dependencies are tracked in the autogen depfile and as
byproducts for correct incremental rebuilds.

This requires Qt 6.13, whose moc supports C++ module units.  With older
Qt nothing can be generated for a module unit, so a meta-object macro
found in one is reported as an error rather than left to fail later in
the compiler or the linker.  Such a unit is no longer handled as an
ordinary source, which also means its moc include directives are not
honored and AUTOUIC does not process it.

moc rejects a meta-object macro in a module implementation unit or a
private module fragment, and AUTOMOC does not work around that.