Add slnf support to dotnet package list

[zivkan]

Bug

Fixes: NuGet/Home#11789

Description

dotnet package list reads solution files via an internal helper method, so this change potentially adds support for solution filters to more than just dotnet package list.

The biggest issue is that there is no public API to load and handle solution filters. See NuGet/Home#11789 (comment) for some more details. NuGet.exe supports slnf by using reflection to call a non-public MSBuild API. I'm using this as precedent as something testing and has been working for years. But it's a risk.

Other info:

• MSBuild's SolutionFile API works with slnx files, so reverted most of the PR that added Microsoft.VisualStudio.SolutionPersistance, since it's no longer needed. This makes a lot of async method into non-async methods. There's a compiler warning about private/internal methods that are async without calling await, and since we treat warnings as errors, it's not optional to deal with them. Hence why there's so much async related changes.
• XPlat.FuncTests that tests MSBuildAPIUtilityTests uses MSBuild.Locator, and therefore tests that validate slnx loading fail if MSBuild is loaded from earlier than 9.0.300, so I bumped our repo's min SDK version to that.

PR Checklist

• Meaningful title, helpful description and a linked NuGet/Home issue
• Added tests
• Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc. the docs on dotnet package list doesn't mention solution filters or slnf, so I don't think any docs need to change.

[zivkan]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[donnie-msft]

the docs on dotnet package list doesn't mention solution filters or slnf, so I don't think any docs need to change.

What about adding something like my bolded words here to the argument definition?

The project, solution file, or solution filter (slnf) to operate on. If not specified, the command searches the current directory for one. If more than one solution or project is found, an error is thrown.