Move libgit2 PDB files to legacy symbol package

Nuget supports two types of symbol packages: [legacy][1] and
[snuget][2]. The latter happens to be the newer implementation. Legacy
symbol packages only exist for compatibility.

It should be noted that, unfortunately, snuget symbol packages do not
support Windows PDBs (the kind you get from C++ code). They only support
portable PDBs (managed). As such, this patch provides support for legacy
symbol package format. Note the following information from the symbol
package MSDN documentation page:

> Native projects, such as C++ projects, produce Windows PDBs instead of
> Portable PDBs. These are not supported by NuGet.org's symbol server.
> Please use Legacy Symbol Packages instead.

Fixes libgit2#111

[1]: https://docs.microsoft.com/en-us/nuget/create-packages/symbol-packages
[2]: https://docs.microsoft.com/en-us/nuget/create-packages/symbol-packages-snupkg

Author: rcdailey

Diff for: buildpackage.ps1

@@ -8,4 +8,9 @@ $buildDate = (Get-Date).ToUniversalTime().ToString("yyyyMMddHHmmss")
 $versionSuffix = ""
 if ($pre.IsPresent) { $versionSuffix = "-pre$BuildDate" }
 
-.\nuget.exe Pack nuget.package\NativeBinaries.nuspec -Version $version$versionSuffix -NoPackageAnalysis
+# Create nuget package + legacy symbol pack
+# See: https://docs.microsoft.com/en-us/nuget/create-packages/symbol-packages
+.\nuget.exe pack nuget.package\NativeBinaries.nuspec `
+    -Symbols `
+    -Version $version$versionSuffix `
+    -NoPackageAnalysis