Rename [Unix|Win32]FileSystemObject.cs files

Rename the following files in preparation for a refactor:
- UnixFileSystemObject.cs -> FileSystemInfo.Unix.cs
- Win32FileSystemObject.cs -> FileSystemInfo.Windows.cs

The renames are being done in their own commit first to help git
maintain source history for the renamed files.

Author: justinvp

src/System.IO.FileSystem/src/System.IO.FileSystem.csproj

@@ -58,12 +58,12 @@
     <Compile Include="Microsoft\Win32\SafeHandles\SafeFileHandle.Windows.cs" />
     <Compile Include="Microsoft\Win32\SafeHandles\SafeFindHandle.Windows.cs" />
     <Compile Include="System\IO\FileStream.Win32.cs" />
+    <Compile Include="System\IO\FileSystemInfo.Windows.cs" />
     <Compile Include="System\IO\PathHelpers.Windows.cs" />
     <Compile Include="System\IO\Win32FileStream.cs" />
     <Compile Include="System\IO\Win32FileStreamCompletionSource.cs" />
     <Compile Include="System\IO\Win32FileSystem.cs" />
     <Compile Include="System\IO\Win32FileSystemEnumerable.cs" />
-    <Compile Include="System\IO\Win32FileSystemObject.cs" />
     <Compile Include="$(CommonPath)\Interop\Windows\Interop.Libraries.cs">
       <Link>Common\Interop\Windows\Interop.Libraries.cs</Link>
     </Compile>
@@ -236,10 +236,10 @@
     <Compile Include="Microsoft\Win32\SafeHandles\SafeFileHandle.Unix.cs" />
     <Compile Include="System\IO\FileStream.Unix.cs" />
     <Compile Include="System\IO\FileSystem.Current.Unix.cs" />
+    <Compile Include="System\IO\FileSystemInfo.Unix.cs" />
     <Compile Include="System\IO\PathHelpers.Unix.cs" />
     <Compile Include="System\IO\UnixFileStream.cs" />
     <Compile Include="System\IO\UnixFileSystem.cs" />
-    <Compile Include="System\IO\UnixFileSystemObject.cs" />
     <Compile Include="$(CommonPath)\Microsoft\Win32\SafeHandles\SafeFileHandle.Unix.cs">
       <Link>Common\Microsoft\Win32\SafeHandles\SafeFileHandle.Unix.cs</Link>
     </Compile>