find_package: Implement UNWIND_INCLUDE

This implements a limited exception mechanism for find_package() via
the UNWIND_INCLUDE keyword.

When package discovery via find_package(UNWIND_INCLUDE) fails the
StateSnapshot is updated to an UNWINDING state. In this state further
calls to find_package() and include() are forbidden. While in the
UNWINDING state, the include() command immediately calls
SetReturnInvoked() whenever it is reached.

The UNWINDING state is reset when a parent call to find_package() is
reached.

Fixes: #26897