`Split-Path` with UNC paths: mind the current provider

When the current PowerShell provider is not FileSystem Split-Path with UNC paths may produce unexpected results or even fail.

Test-1-FileSystem.ps1 - expected result of Split-Path with FileSystem.
Test-2-Function.ps1 - unexpected result of Split-Path with Function.
Test-3.1-Registry.ps1 - Split-Path fails with Registry due to an invalid path.
Test-3.2-Registry.ps1 - Join-Path works with Registry and the above path.
Test-Workaround.ps1 - .NET Path.GetDirectoryName() works.

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