https://github.com/dotnet/roslyn/issues/11512

This is an automated migration of our project files to the new SDK /
PackageReference.  It is done using the tool located here:

https://github.com/jaredpar/ConvertPackageRef

My motivation for doing this is to pick up the new PublicAPI checker
that understands type forwards. In doing so I disabled a bunch of new
warnings that are either broken or not really applicable to Roslyn.
One new warning (don't rethrow exceptions with throw ex;) seemed useful
enough to keep on so I left it on and fixed the one offense in the
codebase.

Fixes #18257

Currently the `CommonCommandLineParser` handles the `/ruleset` switch by reading the .ruleset file and setting the general and specific diagnostic options. However, it doesn't actually save the file path anywhere. This commit now exposes it through the `CommonCommandLineArguments` type, like any other switch that takes a file path.

This change also corrects a subtle issue. As with other switches that take paths, if the `/ruleset` switch appears more than once the last occurrence should "win", and prior occurences should be ignored. However, the dictionary of specific diagnostic options was shared between processing one `/ruleset` and the next; even though the last one should win you could still end up with settings from the other ones. With this change the set of specific diagnostic options is reset with every `/ruleset` processed, ensuring only the last one has any effect.

Technically this is a breaking change. However, you would have to work pretty hard to pass more than one `/ruleset` switch to the compiler. MSBuild only allows one, at most, so you would have to invoke the compiler directly.

Fixes #18188

Refactored IVariableDeclaration to IVariableDeclarationGroup, which allows for multiple declared symbols as part of a single declaration.

With the intention to add them back after the interface and implementation of IHasArgumentsExpression is finalized.