Configuration: allow users to enforce hard spec constraints (#27987)

Spack doesn't have an easy way to say something like "If I build
package X, then I *need* version Y":

* If you specify something on the command line, then you ensure
  that the constraints are applied, but the package is always built
* Likewise if you `spack add X...`` to your environment, the
  constraints are guaranteed to hold, but the environment always
  builds the package
* You can add preferences to packages.yaml, but these are not
  guaranteed to hold (Spack can choose other settings)

This commit adds a 'require' subsection to packages.yaml: the
specs added there are guaranteed to hold. The commit includes
documentation for the feature.

Co-authored-by: Massimiliano Culpo <massimiliano.culpo@gmail.com>

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