Add several new analyzers

[drewnoakes]

Fixes #257
Fixes #287
Fixes #317

New analyzers for:

• Missing [ImportingConstructor]
• Duplicate [ImportingConstructor]
• Duplicate [Import]
• Mismatched C# nullability and AllowDefault

Mostly coded using GitHub Copilot.

[AArnott]

This looks really good. Thanks so much.
There's a lot of code here though, so I have quite a few comments.

[AArnott]

Looks much better. Do you have ideas for resolving the last two comments?

[drewnoakes]

I believe all feedback has been addressed.

[AArnott]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[AArnott]

Questions about InheritedExportAttribute. Ideally you can support that as well. But if you'd rather not, let's just be sure it results in too few diagnostics rather than false positives.

[AArnott]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[AArnott]

@drewnoakes do you have a preference to merge vs. squash?

[AArnott]

I'm gonna gamble that you want the PR completed either way. :)

Thank you very much for your contribution!

[drewnoakes]

Are packages created automatically? Once a version is available, I can try it in a few repos I own and give it a bit more validation.

[AArnott]

Typically yes, they would show up here.
Unfortunately our official pipelines are blocked by lack of optprof data. I've just queued a special optprof run to fix that, so we'll probably get packages built tomorrow night.

[drewnoakes]

Did the optprof run work? I'm not seeing a new analyzers package yet.

[AArnott]

No. Now we're in this weird catch-22 world where the optprof run failed because of the new analyzers.
You can look at the new diagnostics here and decide whether the VS repo has issues that your analyzers are finding or if you're reporting false positives.

[drewnoakes]

Thanks for the pointer. So far I've only seen false positives. Will get a fix together over the next day or so I hope. Let me know if this is blocking anything important and I'll try and accelerate the timeline.

[drewnoakes]

Fixed in #648

[drewnoakes]

Was there any progress on the OptProf run unblocking a new package version? If there are still build failures, I'll take a look (though I'm not sure where to find them).

[AArnott]

I'm working on the OptProf issues this week (I was OOF for the prior two weeks). They aren't build failures. They are failures in Optprof tests that I have no insight into. We're switching OptProf systems to resolve.