concretizer: reuse installs, but assign default values for new builds

Minimizing builds is tricky. We want a minimizing criterion because
we want to reuse the avaialble installs, but we also want things that
have to be built to stick to *default preferences* from the package
and from the user. We therefore treat built specs differently and
apply a different set of optimization criteria to them. Spack's *first*
priority is to reuse what it can, but if it builds something, the built
specs will respect defaults and preferences.

This is implemented by bumping the priority of optimization criteria
for built specs -- so that they take precedence over the otherwise
topmost-priority criterion to reuse what is installed.

The scheme relies on all of our optimization criteria being minimizations.
That is, we need the case where all specs are reused to be better than
any built spec could be. Basically, if nothing is built, all the build
criteria are zero (the best possible) and the number of built packages
dominates. If something *has* to be built, it must be strictly worse
than full reuse, because:

  1. it increases the number of built specs
  2. it must have either zero or some positive number for all criteria

Our optimziation criteria effectively sum into two buckets at once to
accomplish this. We use a `build_priority()` number to shift the
priority of optimization criteria for built specs higher.

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