We would like to record additional information in the find-package
stack, but we don't have the information at the point a stack entry is
created. The most sane way to handle this appears to be making the stack
mutable, at least under specific circumstances. To facilitate this, we
need a mutable stack type.
Refactor cmConstStack into cmStack, with the ability to either allow or
forbid mutation of its data. Re-add cmConstStack as a type alias.
This doesn't yet allow mutating cmFindPackageStack, but it's a necessary
step toward being able to do so.
