In an active concretize environment, support installing one or more
cli specs only if they are already present in the environment. The
`--no-add` option is the default for root specs, but optional for
dependency specs. I.e. if you `spack install <depspec>` in an
environment, the dependency-only spec `depspec` will be added as a
root of the environment before being installed. In addition,
`spack install --no-add <spec>` fails if it does not find an
unambiguous match for `spec`.
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