[JENKINS-60449] Fix and edge case of OptionalExtension blowing up Jenkins

[Vlatombe]

See JENKINS-60449.

Includes binaries from ikedam/dependencytest#2

If you have an extension like this

@OptionalExtension(requirePlugins = {"dependee"})
public class OptionalDependerExtension {
    private static void foo(Dependee d) {

    }
}

and dependee is not loaded initially, but you load it dynamically later, this blows up, because when Guice does a method scan and can't resolve Dependee because it is not yet in classloader at the time of scanning.

This PR can be reviewed commit by commit.

• Introduce a failing test
• Introduce functional changes fixing the test

Proposed changelog entries

• Entry 1: Fix an edge case of loading optional dependencies that cause Jenkins to blow up
• ...

Submitter checklist

• JIRA issue is well described
• Changelog entry appropriate for the audience affected by the change (users or developer, depending on the change). Examples
  * Use the Internal: prefix if the change has no user-visible impact (API, test frameworks, etc.)
• Appropriate autotests or explanation to why this change has no tests
• For dependency updates: links to external changelogs and, if possible, full diffs

Desired reviewers

@mention

[Vlatombe]

@jglick Updated the PR with a simpler diff (no more refactoring, and a new failing test instead of updating an existing one)

[halkeye]

This is so over my head, so as a core pr reviewer I'm going to abstain from this one but its cool to try to figure out what's going on

+1 for tests

[fcojfernandez]

IIUC, code looks good to me

[oleg-nenashev]

Looks fine though it would be great to run PCT against the patched version

[jglick]

Guice does a method scan and can't resolve Dependee

By the way: google/guice#1283