Change protected overload of PackageManager.InstallPackage to be public

[glennblock]

scriptcs relies on NuGet.Core. We started having issues recently as whenever we installed NuGet Packages that also support .NET Standard / .NET Core, we started getting all the core packages, even though they are not needed.

Looking through the code I noticed that targetFramework is always forced to null in the public InstallPackage overloads. This meant I had to create my own derived PackageManager class just to expose the protected overload. You can see I did that here. For users of NuGet.Core, I'd argue it's reasonable for them to be able to force the target version without having to derive.

[dnfclas]

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[dnfclas]

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