This PR adds a 'concretize' entry to an environment's spec.yaml file
which controls how user specs are concretized. By default it is
set to 'separately' which means that each spec added by the user is
concretized separately (the behavior of environments before this PR).
If set to 'together', the environment will concretize all of the
added user specs together; this means that all specs and their
dependencies will be consistent with each other (for example, a
user could develop code linked against the set of libraries in the
environment without conflicts).
If the environment was previously concretized, this will re-concretize
all specs, in which case previously-installed specs may no longer be
used by the environment (in this sense, adding a new spec to an
environment with 'concretize: together' can be significantly more
expensive).
The 'concretize: together' setting is not compatible with Spec
matrices; this PR adds a check to look for multiple instances of the
same package added to the environment and fails early when
'concretize: together' is set (to avoid confusing messages about
conflicts later on).
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