Fix netstd2 issue on XM full by expanding facades the same as Modern

[chamons]

• The idea is to force Full and Modern to expand facades the same way. That way, we get the same, working behavior.
• f79f2e4 was not sufficient, even though it matched XI, because of the difference between XI (and Modern) and what Full was doing.

[rolfbjarne]

Do you have a test for this?

[chamons]

Yes, but they are disabled due to https://bugzilla.xamarin.com/show_bug.cgi?id=53164

[monojenkins]

Build failure

[radical]

I would add to the commit message, that
f79f2e4 did #2685 (comment)
and ..

And that was problematic because it was expanding the netstandard facades from `Microsoft.NET.Build.Extensions`
in the `ImplicitlyExpandNETStandardFacades` target.
But we want to build against XM's bundled facades *only*. So we disable the ns facades completely
by setting `$(ImplicitlyExpandNETStandardFacades) = false`.

But now we are in the situation where a XM/Full project referencing a ns project might fail to build
because of a missing `netstandard.dll` reference! And this same case was fixed for XM/Modern projects in
https://github.com/xamarin/xamarin-macios/pull/2643 . So, we enable the use of that for XM/Full projects too
through `Xamarin.Mac.msbuild.targets`.

[chamons]

Also add link to https://bugzilla.xamarin.com/show_bug.cgi?id=59474

[chamons]

Test failure: https://bugzilla.xamarin.com/show_bug.cgi?id=59654