We have two ways to concretize now:
* `spack concretize` concretizes only the root specs that are not concrete in the environment.
* `spack concretize -f` eliminates all cached concretization data and reconcretizes the *entire* environment.
This PR adds `spack deconcretize`, which eliminates cached concretization data for a spec. This allows
users greater control over what is preserved from their `spack.lock` file and what is reused when not
using `spack concretize -f`. If you want to update a spec installed in your environment, you can call
`spack deconcretize` on it, and that spec and any relevant dependents will be removed from the lock file.
`spack concretize` has two options:
* `--root`: limits deconcretized specs to *specific* roots in the environment. You can use this to
deconcretize exactly one root in a `unify: false` environment. i.e., if `foo` root is a dependent
of `bar`, both roots, `spack deconcretize bar` will *not* deconcretize `foo`.
* `--all`: deconcretize *all* specs that match the input spec. By default `spack deconcretize`
will complain about multiple matches, like `spack uninstall`.
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