Concretize reuse: reuse specs from environment (#45139)

The already concrete specs in an environment are now among the reusable specs for the concretizer.

This includes concrete specs from all include_concrete environments.

In addition to this change to the default reuse, `environment` is added as a reuse type for 
the concretizer config. This allows users to specify:

spack:
  concretizer:
    # Reuse from this environment (including included concrete) but not elsewhere
    reuse:
      from:
      - type: environment
    # or reuse from only my_env included environment
    reuse:
      from:
      - type:
          environment: my_env
    # or reuse from everywhere
    reuse: true

If reuse is specified from a specific environment, only specs from that environment will be reused.
If the reused environment is not specified via include_concrete, the concrete specs will be retried
at concretization time to be reused.

Signed-off-by: Ryan Krattiger <ryan.krattiger@kitware.com>
Co-authored-by: Gregory Becker <becker33@llnl.gov>

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