A stack entry's storage may be shared by other copies, so mutation is
incompatible with value semantics. We've migrated the motivating use
case to another approach.
Revert commit b3873b8272 (cmFindPackageStack: Allow controlled mutation,
2025-07-29, v4.2.0-rc1~438^2) and commit f2bdc2176f (cmStack: New,
mutable stack class, 2025-07-29, v4.2.0-rc1~438^2~1). Record their
parent as a second parent of this commit so `git blame` can see the
original history of the restored content.
561ece2407