[Xamarin.Android.Build.Tasks] <ResolveAssemblies/> and ref assemblies

[jonathanpeppers]

Fixes: #3045

In VS 2019 16.1 Preview, a project using Microsoft.Extensions.Http
fails to build with:

C:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\MSBuild\Xamarin\Android\Xamarin.Android.Common.targets(2146,5): error MSB4018: The "LinkAssemblies" task failed unexpectedly.
System.IO.FileNotFoundException: Could not load assembly 'System.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'. Perhaps it doesn't exist in the Mono for Android profile?
    File name: 'System.Runtime.dll'
    at Java.Interop.Tools.Cecil.DirectoryAssemblyResolver.Resolve(AssemblyNameReference reference, ReaderParameters parameters)
    at Java.Interop.Tools.Cecil.DirectoryAssemblyResolver.Resolve(AssemblyNameReference reference)
    at Mono.Cecil.MetadataResolver.Resolve(TypeReference type)
    at Mono.Cecil.ModuleDefinition.Resolve(TypeReference type)
    at Mono.Cecil.TypeReference.Resolve()
    at Java.Interop.Tools.Cecil.TypeDefinitionRocks.<GetTypeAndBaseTypes>d__1.MoveNext()
    at System.Linq.Enumerable.Any[TSource](IEnumerable`1 source, Func`2 predicate)
    at Java.Interop.Tools.Cecil.TypeDefinitionRocks.IsSubclassOf(TypeDefinition type, String typeName)
    at MonoDroid.Tuner.FixAbstractMethodsStep.ProcessAssembly(AssemblyDefinition assembly)
    at Mono.Linker.Steps.BaseStep.Process(LinkContext context)
    at Mono.Linker.Pipeline.Process(LinkContext context)
    at MonoDroid.Tuner.Linker.Process(LinkerOptions options, ILogger logger, LinkContext& context)
    at Xamarin.Android.Tasks.LinkAssemblies.Execute(DirectoryAssemblyResolver res)
    at Xamarin.Android.Tasks.LinkAssemblies.Execute()
    at Microsoft.Build.BackEnd.TaskExecutionHost.Microsoft.Build.BackEnd.ITaskExecutionHost.Execute()
    at Microsoft.Build.BackEnd.TaskBuilder.<ExecuteInstantiatedTask>d__26.MoveNext() [C:\Temp\ResolveAssembliesSystemRuntime\AndroidApp1\AndroidApp1.csproj]

The problem being, that the <ResolveAssemblies/> MSBuild task didn't
return System.Runtime.dll in @(ResolvedAssemblies).

What further complicated matters, was that the same project was
working with latest xamarin-android/master (16.2)? I was only able to
reproduce the problem in a test after some work to create a project
that only has a <PackageReference/> to Microsoft.Extensions.Http
with no other references at all. I had to do some MSBuild trickery
to remove Java.Interop and System.Runtime references.

Looking at the dependency tree in the failing test,
System.Runtime.CompilerServices.Unsafe.dll should be reporting a
reference to System.Runtime.dll. Through debugging, I saw it only
had a reference to netstandard.dll?

Then I realized there were two assemblies:

~\.nuget\packages\system.runtime.compilerservices.unsafe\4.5.1\ref\netstandard2.0\System.Runtime.CompilerServices.Unsafe.dll
~\.nuget\packages\system.runtime.compilerservices.unsafe\4.5.1\lib\netstandard2.0\System.Runtime.CompilerServices.Unsafe.dll

The "ref" assembly, references netstandard.dll and the "lib"
assembly (that we should be using) references System.Runtime.dll.

Through further debugging, I found two bugs here:

• We shouldn't call resolver.AddSearchDirectory until after we've
  checked if the top-level assembly is a reference assembly. We don't
  want a reference assembly in the search paths!
• MetadataResolver should key its cache on the resolved path to the
  assembly. This allows <ResolveAssemblies/> to open two different
  instances of System.Runtime.CompilerServices.Unsafe.

[jonathanpeppers]

I'm off tomorrow, but when I'm back I want to test cherry-picking this to d16-1 and be sure the problem will be fixed there.

/cc @brendanzagaeski

[dellis1972]

This look good. One test failure is not related to this PR, and one windows its the AOT stuff which is still failing. Again not related to this PR.