(NuGet#9) Fix path separator in nuspec file element

Converts the path separator character in the nuspec file source to the
character that is correct for the OS that is running the packing
operation. Changes "\" to "/" on Linux and MacOS systems, and
changes "/" to "\" on Windows systems.

Keeps chocolatey/choco#2166 fixed

Based on this commit:
chocolatey/nuget-chocolatey@979a47d

src/NuGet.Core/NuGet.Packaging/PackageCreation/Authoring/ManifestReader.cs

@@ -456,14 +456,23 @@ private static List<ManifestFile> ReadFilesList(XElement xElement)
                 var target = file.GetOptionalAttributeValue("target").SafeTrim()?.TrimStart(slashes);
                 var exclude = file.GetOptionalAttributeValue("exclude").SafeTrim();
 
+                //////////////////////////////////////////////////////////
+                // Start - Chocolatey Specific Modification
+                //////////////////////////////////////////////////////////
+
                 // Multiple sources can be specified by using semi-colon separated values.
+                char separator = Path.DirectorySeparatorChar;
                 files.AddRange(srcElement.Value.Trim(';').Split(';').Select(s =>
                     new ManifestFile
                     {
-                        Source = s.SafeTrim(),
+                        Source = s.SafeTrim().Replace('/', separator).Replace('\u005c', separator),
                         Target = target,
                         Exclude = exclude
                     }));
+
+                //////////////////////////////////////////////////////////
+                // End - Chocolatey Specific Modification
+                //////////////////////////////////////////////////////////
             }
             return files;
         }