The reusing concretizer minimizes builds, but it still preserves
defaults from packages and preferences while doing that. We can be
more aggressive by making minimization the top priority, at the
expense of "weird" concretizations.
This can be advantageous: if you write your packages as explicitly
as possible, then you can use that with `--minimal` to get the
smallest possible package configuration (at least in terms of the
number of packages in the build). Conversely, you can use minimal
concretization as kind of a worst case to ensure that you have the
"right" constraints on your dependencies.
Example for intuition: `cmake` can optionally build without openssl, but
it's enabled by default because many builds use that functionality. Using
`minimal: true` will build `cmake~openssl` unless the user asks for
`cmake+openssl` explicitly.
- [x] add `minimal` option to `concretizer.yaml`
- [x] add `--minimal` CLI option to concretizer arguments
- [x] wire everything up
- [x] add some tests
3985b307a1