Fixes https://github.com/dotnet/project-system/issues/1918.

The language service implements `IVsSolutionLoadEvents.OnBeforeLoadProjectBatch` and `IVsSolutionLoadEvents.OnAfterLoadProjectBatch` to support asynchronous solution loads. While a batch is in progress we try to avoid pushing added projects to the workspace and instead store them in a list. When the batch is complete we may push all of those projects to the workspace at once.

The current implementation has a memory leak. Project may be added outside of a pair of `OnBeforeLoadProjectBatch`/`OnAfterLoadProjectBatch` calls, and it is possible for projects to be added without those methods ever being called. However, during a solution load adding a project always causes us to put it in the list of batched projects. Since we only clear the list on calls to `OnBeforeLoadProjectBatch` or `OnAfterLoadProjectBatch` the user could close the project or even the whole solution and we could still be holding on to one or more projects and all of their associated memory.

The fix here is to add a `bool` to track whether or not we're batching projects, and only put an added project in the list if we are.

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