"spack install foo" no longer adds package "foo" to the environment
(i.e. to the list of root specs) by default: you must specify "--add".
Likewise "spack uninstall foo" no longer removes package "foo" from
the environment: you must specify --remove. Generally this means
that install/uninstall commands will no longer modify the users list
of root specs (which many users found problematic: they had to
deactivate an environment if they wanted to uninstall a spec without
changing their spack.yaml description).
In more detail: if you have environments e1 and e2, and specs [P, Q, R]
such that P depends on R, Q depends on R, [P, R] are in e1, and [Q, R]
are in e2:
* `spack uninstall --dependents --remove r` in e1: removes R from e1
(but does not uninstall it) and uninstalls (and removes) P
* `spack uninstall -f --dependents r` in e1: will uninstall P, Q, and
R (i.e. e2 will have dependent specs uninstalled as a side effect)
* `spack uninstall -f --dependents --remove r` in e1: this uninstalls
P, Q, and R, and removes [P, R] from e1
* `spack uninstall -f --remove r` in e1: uninstalls R (so it is
"missing" in both environments) and removes R from e1 (note that e1
would still install R as a dependency of P, but it would no longer
be listed as a root spec)
* `spack uninstall --dependents r` in e1: will fail because e2 needs R
Individual unit tests were created for each of these scenarios.
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