Make sure bundle update <specific_gems> can always update to the latest resolvable version of each requested gem

[deivid-rodriguez]

What was the end-user or developer problem that led to this PR?

Sometimes bundle update foo won't actually update foo, even if a clean bundle install will resolve to a higher version of foo than the one currently locked.

The problem is that other locked dependencies may be setting some constraints on dependencies of foo, preventing it from being upgraded. These locked dependencies are not present on a clean bundle install.

What is your fix for the problem, implemented in this PR?

My fix is, if bundle update foo is requested, and initial try did not update foo, try unlocking other gems, even if they are not dependencies of foo. Keep doing this until foo updates, or it's not possible to unlock more gems.

Fixes #3310.
Fixes #5463.
Fixes #7176.

Supersedes #5520.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

[deivid-rodriguez]

It took me a while to figure this out, but I think I found the best approach to tackle this 🎉.

I realized that the initial approach here was too naive and could result in unnecessary updates unrelated to updating the requested gems. I had also only implemented the solution for bundle update <specific_gem>, but not for multiple gems.

I figured out a better algorithm which is the following. When bundle update <specific_gems> is run, first do a full bundle update and parse the resulting versions of the specifically requested gems. Then add additional constraints to the resolver to force those versions. Adding these constraints may result into conflicts, however, our resolver already has logic to resolve conflicts by incrementally unlocking conflicting gems as necessary, so this just works I think!

As a nice extra, this approach should make it easier to implement further improvements, like #3290.