add a flag for relaxing the way we use graphs to constrain version resolution

[dhellmann]

We've added a flag to tell the bootstrap logic to use a previously produced graph file as a cache for exact versions of packages that it should use when resolving to allow repeatable builds. We should extend that with a flag that allows "compatible" updates to those packages, to make it easier to move from builds that are completely unbounded, to builds that only take some updates, to builds that require manual updates of all dependencies.

When this new flag is provided, fromager should set up a constraint based on the historical resolution using the ~= operator instead of ==. This would allow patch version updates of dependencies to be included.