Annotate System.Net.WebProxy for nullable reference types

[stephentoub]

Depends on #31794
Contributes to #2339

(Also fixed a capitalization typo in some internal members.)

cc: @buyaa-n, @eiriktsarpalis, @safern, @eerhardt

[jaredpar]

@mmitche looks like this PR is hitting the same NuGet cache bug you hit. Did you file the issue to track that?

 src/mono/netcore/nuget/packages.builds(0,0): error : System.AggregateException: One or more errors occurred. (Could not find a part of the path '/tmp/NuGetScratch/fddf3c7a91d54497a7b711c945fe60d3/b8178b951e2f4151a1758acd7ba7326f.proj.nuget.dgspec.json'.)
 ---> System.IO.DirectoryNotFoundException: Could not find a part of the path '/tmp/NuGetScratch/fddf3c7a91d54497a7b711c945fe60d3/b8178b951e2f4151a1758acd7ba7326f.proj.nuget.dgspec.json'.
   at Interop.ThrowExceptionForIoErrno(ErrorInfo errorInfo, String path, Boolean isDirectory, Func`2 errorRewriter)
   at Microsoft.Win32.SafeHandles.SafeFileHandle.Open(String path, OpenFlags flags, Int32 mode)
   at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, FileOptions options)
   at System.IO.FileStream..ctor(String path, FileMode mode)
   at NuGet.ProjectModel.DependencyGraphSpec.Save(String path)
   at NuGet.Commands.NoOpRestoreUtilities.PersistDGSpecFile(DependencyGraphSpec spec, String dgPath, ILogger log)
   at NuGet.Commands.RestoreCommand.EvaluateCacheFile()
   at NuGet.Commands.RestoreCommand.ExecuteAsync(CancellationToken token)
   at NuGet.Commands.RestoreRunner.ExecuteAsync(RestoreSummaryRequest summaryRequest, CancellationToken token)
   at NuGet.Commands.RestoreRunner.CompleteTaskAsync(List`1 restoreTasks)
   at NuGet.Commands.RestoreRunner.RunWithoutCommit(IEnumerable`1 restoreRequests, RestoreArgs restoreContext)
   --- End of inner exception stack trace ---
   at System.Threading.Tasks.Task`1.GetResultCore(Boolean waitCompletionNotification)
   at Microsoft.Build.NuGetSdkResolver.NuGetSdkResolver.NuGetAbstraction.GetSdkResult(SdkReference sdk, Object nuGetVersion, SdkResolverContext context, SdkResultFactory factory)
src/mono/netcore/nuget/packages.builds(0,0): error MSB4236: The SDK 'Microsoft.Build.Traversal' specified could not be found.
(NETCORE_ENGINEERING_TELEMETRY=Build) Build failed (exit code '1').
Bash exited with code '1

[eerhardt]

LGTM.

[mmitche]

@mmitche looks like this PR is hitting the same NuGet cache bug you hit. Did you file the issue to track that?

 src/mono/netcore/nuget/packages.builds(0,0): error : System.AggregateException: One or more errors occurred. (Could not find a part of the path '/tmp/NuGetScratch/fddf3c7a91d54497a7b711c945fe60d3/b8178b951e2f4151a1758acd7ba7326f.proj.nuget.dgspec.json'.)
 ---> System.IO.DirectoryNotFoundException: Could not find a part of the path '/tmp/NuGetScratch/fddf3c7a91d54497a7b711c945fe60d3/b8178b951e2f4151a1758acd7ba7326f.proj.nuget.dgspec.json'.
   at Interop.ThrowExceptionForIoErrno(ErrorInfo errorInfo, String path, Boolean isDirectory, Func`2 errorRewriter)
   at Microsoft.Win32.SafeHandles.SafeFileHandle.Open(String path, OpenFlags flags, Int32 mode)
   at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, FileOptions options)
   at System.IO.FileStream..ctor(String path, FileMode mode)
   at NuGet.ProjectModel.DependencyGraphSpec.Save(String path)
   at NuGet.Commands.NoOpRestoreUtilities.PersistDGSpecFile(DependencyGraphSpec spec, String dgPath, ILogger log)
   at NuGet.Commands.RestoreCommand.EvaluateCacheFile()
   at NuGet.Commands.RestoreCommand.ExecuteAsync(CancellationToken token)
   at NuGet.Commands.RestoreRunner.ExecuteAsync(RestoreSummaryRequest summaryRequest, CancellationToken token)
   at NuGet.Commands.RestoreRunner.CompleteTaskAsync(List`1 restoreTasks)
   at NuGet.Commands.RestoreRunner.RunWithoutCommit(IEnumerable`1 restoreRequests, RestoreArgs restoreContext)
   --- End of inner exception stack trace ---
   at System.Threading.Tasks.Task`1.GetResultCore(Boolean waitCompletionNotification)
   at Microsoft.Build.NuGetSdkResolver.NuGetSdkResolver.NuGetAbstraction.GetSdkResult(SdkReference sdk, Object nuGetVersion, SdkResolverContext context, SdkResultFactory factory)
src/mono/netcore/nuget/packages.builds(0,0): error MSB4236: The SDK 'Microsoft.Build.Traversal' specified could not be found.
(NETCORE_ENGINEERING_TELEMETRY=Build) Build failed (exit code '1').
Bash exited with code '1

I had not.

[jaredpar]

@mmitche eventually figured that out. Ended up filing this issue

NuGet/Home#9199

[safern]

LGTM