concretizer: rework operating system semantics for installed packages

The OS logic in the concretizer is still the way it was in the first version.
Defaults are implemented in a fairly inflexible way using straight logic. Most
of the other sections have been reworked to leave these kinds of decisions to
optimization. This commit does that for OS's as well.

As with targets, we optimize for target matches. We also try to optimize for
OS matches between nodes. Additionally, this commit adds the notion of
"OS compatibility" where we allow for builds to depend on binaries for certain
other OS's. e.g, for macos, a bigsur build can depend on an already installed
(concrete) catalina build. One cool thing about this is that we can declare
additional compatible OS's later, e.g. CentOS and RHEL.

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