Support loading external annotations from NuGet package

[bkoelman]

Allowing users to Install-Package Jetbrains.ExternalAnnotations makes it possible for users to build the source tree without having Resharper installed. This also simplifies running on build servers, which usually do not have Resharper installed.

Giving this location highest priority will also result in producing the same diagnostics for all developers, regardless of their installed version of Resharper.

To enable this, we should look for external annotations next to the .sln file that matches .\packages\JetBrains.ExternalAnnotations.*\DotFiles\ExternalAnnotations. When external annotations are found there, we should not fallback to the other locations.

How to find the .sln folder from within the running analyzer is to be investigated.

[bkoelman]

Complications:

• The name and location of the packages folder can be customized.
• csproj-based .NET Core projects do not have a packages.json file, but have PackageReference elements in the project file.
• Based on the location of the loaded analyzer dll, we can try to probe. But in VS2017, all packages are in a shared location, so the package may exist there due to another solution that depends on it. Not very reliable.
• Using the location of the analyzer dll would only work for the NuGet variant of this analyzer.

[bkoelman]

These is currently no API available to make this possible. See https://github.com/dotnet/roslyn/issues/15977#issuecomment-268620263.