Use McMaster.NETCore.Plugins for assembly loading

[amis92]

• Engine uses McMaster.NETCore.Plugins package to resolve assemblies and dependencies, enabling generators to reference additional dependencies! 🎉
• Tasks deprecate GeneratorAssemblySearchPaths usage
• Tasks use CodeGenerationRoslynPlugin ItemGroup instead, which contains concrete assembly paths (instead of containing folder paths, as was previously)
• Engine targets netcoreapp2.0 to reference McMaster package
• Tests use Amadevus.RecordGenerator NuGet generator for back-compat checks
• Tests.Generators use Bogus NuGet for NuGet dependency resolution check
• CodeGeneration.Roslyn.Plugin.Sdk MSBuild project Sdk created, to help build and package plugins correctly.
• Migrate to use VS2019/.NET Core SDK v3.1
• Rewritten Readme with a simpler demo and more advanced scenarios

Added https://github.com/AArnott/CodeGeneration.Roslyn/wiki/Migrations#v07 to help with migrations.

closes #114

CC @Pzixel

TODO:

• [x] Update documentation, README etc.

[Pzixel]

Cool. The only bad thing about that is that I probably changing my job so I won't need a codegen soon. Waited for too long 😄

Awesome work guys, anyway. Gonna update my library if it fits. I will check this when it gets into the nuget.

[AArnott]

Please do update the README as you say though.

[tedvanderveen]

Great work guys, this should fix the issue I'm facing right now!

[Pzixel]

And it still open. Guys, that issue blocks a lot of good stuff. Consider revisiting it soon? CC @amis92

[Pzixel]

I wonder if this is going to merge before 2020

[amis92]

I'd like to announce I'm back, working on it. I have to admit, the "your code from 3 months ago could be written by someone else" saying is so true.

If someone wants to help, don't hesitate. I'll probably run some tests, try to touch up documentation to the best of my abilities, and push a beta release since I really need some external input and testing.

[Pzixel]

I'd like to help but I won't be able to if there won't be a nuget package with it.

My primary OS is windows so I only can build in docker. And the only sane way of running it in docker is pull items from nuget. Packing nuget locally and copying it via Dockerfile doesn't seem to be an easy task.

[amis92]

📝 I'm using MSBuildWarningsAsMessages feature of v15.0 (https://github.com/microsoft/msbuild/pull/1928).

[amis92]

I've added a Plugin.Sdk (first idea #113) to the mix, and I think quite extensive documentation in the readme.

@AArnott want to play with it, or merge and push out as a beta to let the people play?

[AArnott]

@amis92 Thanks again for putting this together. I'm not sure when I'll have time to play with it. Maybe next week. If I don't get back to you early next week feel free to complete anyway.

[amis92]

[amis92]

@Pzixel

[Pzixel]

I wonder if this is going to merge before 2020

Well, it didn't... :)

guess my origin

Thanks for PR, I'l try to manage to get to it soon.