Better version specific relationships at install and upgrade

[HebaruSan]

I've been sitting on these changes for a while because I wasn't quite happy with the implementation, but I think it's about as good as it's going to get for now, and I want to get it off my plate to focus on other issues.

Problem

If you install a mod that has a version-specific dependency on an older version of another mod, CKAN will appear to treat that other mod as upgradeable, but the upgrade will fail if attempted. Intermediate versions that could satisfy all requirements will not be discovered automatically, but may be selected manually by the user.

E.g., RP-0 recently did this:

depends:
  - name: RealismOverhaul
    max_version: v15.99.0.0

Versions after that are meant to be used only with RP-1. But RealismOverhaul will still show up as upgradeable, and trying to upgrade it will fail.

Cause

• AvailableModule.DependsAndConflictsOK doesn't check version requirements of other modules that depend on the given module
• In general, mod upgradeability is treated as a property of the individual mod, when it really needs to be a collective property of groups of mods that are installed or being installed.

Changes

• Now reverse dependencies of installed mods are considered when looking for available versions
• IRegistryQuerier.HasUpdate now accepts a list of other modules that are used to limit the compatible versions of the given mod
• A new function IRegistryQuerier.CheckUpgradeable is created to provide more complex upgrading logic; it first finds the latest available versions of all installed mods, then uses their relationship constraints to roll back one another until a consistent changeset is generated. This function is now used by:
  • GUI's upgrade checkboxes, via setting GUIMod.HasUpdate from outside GUIMod
  • The changeset is updated based on what upgrades are actually allowed
  • ckan upgrade --all
  • ckan list's status characters
• Mods with a "held" label are treated as not upgradeable, so they're excluded from that initial step of looking for the latest versions. When you add or remove labels or edit a label, we recalculate which upgrade checkboxes are shown.
• Several tests are added to ensure ckan upgrade works as intended

Fixes #3849. Fixes #3945.