We support non-compiled `SOURCES` on `INTERFACE` libraries, and also
support `CXX_MODULES` on *imported* `INTERFACE` libraries (via synthetic
targets that compile module interface units). However, we do not
support `CXX_MODULES` on non-imported `INTERFACE` libraries because
there is no place to hold module interface unit's object files for their
module initializers. Previously this was not explicitly rejected, and
so was diagnosed only by "CMake Internal Error" messages due to
assumption violations in the implementation.
Fixes: #26524
Co-authored-by: Ben Boeckel <ben.boeckel@kitware.com>
