[Vcxproj] Improve NuGet handling in packages.config mode & add necessary logic for package reference mode

[zyzyz]

1. Improve .targets import logic when using NuGet packages.config mode

Old Behaviour: /build/native/*.targets and /build/*.targets are both imported.

Problem: in common, /build/native/*.targets for C++ project is firstly chosen if presented, old logic will make /build/*.targets also be imported, and in before of /build/native/*.targets, causing troubles.
e.g. Microsoft.WindowsAppSDK

Current Behaviour: /build/native/*.targets import is made before /build/*.targets , it will check if /build/native/*.targets
is not presented, then do the /build/*.targets import.

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2. Add support codes for NuGet package reference mode in .vcxproj

The current NuGet support for C++ project is incomplete.

• .props file import is not supported.
• lack methods to determine if .targets and .props files are initially presented in order to generate correct error check.
• Irregular path to .targets and .props files are not supported.
  • Like the problem discussed below (yet providing a configurable custom path is not a good solution)
    [Vcxproj] Support explicit path to NuGet .targets, and clean up existing packages.config if no longer needed #390
• All other downsides of packages.config mode (cannot restore dependencies, etc.)

here and NuGet/NuGet.Client#3145 have shown the way to force package reference mode for .vcxproj.
To achieve this in Sharpmake, it requires

1. A custom configuration of Directory.Build.props
2. <PackageReference> element generation support inside Sharpmake

This PR contains the 2. part.

I haven't figure out the best way to automatically handle 1., but since Directory.Build.props is originally a thing that users manually add to their projects, so it should be no harm to let them also add that custom configuration manually (for now?)

P.S. the default NuGet mode for C++ project still keeps as packages.config.

[jspelletier]

Hi
We've started to check this PR. We ran regression tests on our big engines and it doesn't change anything(we don't use the features that this PR changes).

The next step is we need some samples using this if you ever want this to land eventually.

Thanks.