[build] Sanitize Microsoft.Build projects

[pjcollins]

Context: #3928
Context: https://github.com/xamarin/monodroid/pull/1048

Attempts to clean up and modernize various projects to help break up the
scope of change required to convert them to .NET Standard libraries.

Updates the shared MSBuildReferences.projitems file to instead include
NuGet references to the required Microsoft.Build.* assemblies. The
Mono.Posix.NETStandard, Xamarin.Build.AsyncTask, and Xamarin.LibZipSharp
NuGet references have also been moved into this file to make it easier
to update versions in one place as needed. We're only using the compile
time assets from the MSBuild NuGet packages, which ensures that their
content is not copied to the output directory. An additional ItemGroup
has been added to this file to copy the required assemblies from the system
to the output directory so that Xamarin.Android.Build.Tests can run against
them. These changes are required because the MSBuild assemblies coming
from NuGet do not yet fully support running on Mono.

All projects which reference MSBuildReferences.projitems have been
converted from the old packages.config to the new PackageReference
NuGet reference style.

The Azure Pipeline build job has been updated to completely build
Xamarin.Android-Tests.sln with xabuild, rather than only building select
projects. This was done to mirror our Jenkins build and to ensure that
make all-tests continues to work moving forward.

.NET Core and NuGetCommand tasks have been bumped to bring
in a newer version of NuGet with better PackageReference support.

NuGet restore will be skipped if any issues occur when calculating the
dependency graph for the solution or project that it is operating on.
A lot of our sources attempt to import props/targets/projitems files
that are generated by the build, and these should be conditionally
imported so that NuGet restore attempts can succeed before these files
are generated. This should also help when loading these projects in
VS or VS Mac.

Example NuGet restore error:

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Build.Tasks.targets(8,3): error MSB4019: The imported project "bin/BuildRelease/ProfileAssemblies.projitems" was not found. Confirm that the expression in the Import declaration "../../bin/BuildRelease/ProfileAssemblies.projitems" is correct, and that the file exists on disk.
/var/folders/pp/x88nqyvd0h18ttl996vv2c3w0000gn/T/NuGetScratch/ok90j1ok.3t3.nugetrestore.targets(266,5): warning : Skipping restore for project 'src/Xamarin.Android.Build.Tasks/Xamarin.Android.Build.Tasks.csproj'. The project file may be invalid or missing targets required for restore.

I've also moved the NuGet.exe that is copied and used by tests into it's own
folder to avoid a crash on macOS which would occur when NuGet.exe is
invoked from the same folder as Xunit.NetCore.Extensions.dll:

mono bin/TestRelease/NuGet.exe

VTable setup of type Xunit.NetCore.Extensions.SkippedTestCase failed
System.TypeLoadException: VTable setup of type Xunit.NetCore.Extensions.SkippedTestCase failed
  at (wrapper managed-to-native) System.RuntimeType.GetPropertiesByName_native(System.RuntimeType,intptr,System.Reflection.BindingFlags,System.RuntimeType/MemberListType)
  at System.RuntimeType.GetPropertiesByName (System.String name, System.Reflection.BindingFlags bindingAttr, System.RuntimeType+MemberListType listType, System.RuntimeType reflectedType) [0x0001b] in <7d0bc631cadd4a07bf74717847e90cfa>:0
  at System.RuntimeType.GetPropertyCandidates (System.String name, System.Reflection.BindingFlags bindingAttr, System.Type[] types, System.Boolean allowPrefixLookup) [0x00010] in <7d0bc631cadd4a07bf74717847e90cfa>:0
  at System.RuntimeType.GetProperties (System.Reflection.BindingFlags bindingAttr) [0x00000] in <7d0bc631cadd4a07bf74717847e90cfa>:0
  at System.ComponentModel.Composition.AttributedModel.AttributedPartCreationInfo+<GetExportMembers>d__32.MoveNext () [0x000eb] in <5e20d4aa8c6c44a89bf1aeb3c06f166c>:0
  at System.Linq.Enumerable.Any[TSource] (System.Collections.Generic.IEnumerable1[T] source) [0x00015] in <e22351e395f44d7b8c4860fca2dcee71>:0
  ...

[pjcollins]

There's a macOS specific issue in Xamarin.Android.Build.Tests that I'm looking at now.