[Xamarin.Android.Build.Tasks] Better support for netstandard libraries. #1356

Fixes dotnet#1154, dotnet#1162 Netstandard packages sometimes ship with both reference and implementation assemblies. The Nuget build task `ResolveNuGetPackageAssets` only resolves the `ref` version of the assemblies. There does not seem to be away way to FORCE Nuget to resolve the lib one. How .net Core manages to do this is still a mistery. That said the Nuget `ResolveNuGetPackageAssets` does give us a hint as to how to use the `project.assets.json` file to figure out what `lib` version of the package we should be including. This commit reworks `ResolveAssemblies` to attempt to map the `ref` to a `lib` if we find a Referenece Assembly. Historically we just issue a warning (which will probably be ignored), but now we will use the `project.assets.json` file to find the implementation version of the `ref` assembly. We need to be using `Nuget.ProjectModel` since it an API for querying the `project.assets.json`. We make use of the Nuget build properties `$(ProjectLockFile)` for the location of the `project.assets.json` , `$(NuGetPackageRoot)` for the root folder of the Nuget packages and `$(NuGetTargetMoniker)` for resolving which `TargetFrameworks` we are looking for. All of these properties should be set by Nuget. If they are not we should fallback to the default behaviour and just issue the warning. { "version": 3, "targets": { "MonoAndroid,Version=v8.1": { "System.IO.Packaging/4.4.0": { "type": "package", "compile": { "ref/netstandard1.3/System.IO.Packaging.dll": {} }, "runtime": { "lib/netstandard1.3/System.IO.Packaging.dll": {} } }, } } } The code above is a cut down sample of the `project.assets.json`. So our code will first resolve the `targets`. We use `$(NuGetTargetMoniker)` to do this. For an android project this should have a value of `MonoAndroid,Version=v8.1`. Note we do NOT need to worry about the version here. When Nuget restores the packages it creates the file so it will use the correct version. Next we try to find the `System.IO.Packaging`. We need to look at the `lockFile.Libraries` to get information about the install path in the Nuget folder, and then `target.Libraries` to pick out the `runtime` item. Once we have resolved the path we need to then combine that with the `$(NuGetPackageRoot)` to get the full path to the new library. If at any point during all of this code we don't get what we expect (i.e a null) we should abort and just issue the warning. The only real concern with this is if the format of the `project.assets.json` file changes. It is not ment to be edited by a human so there is the possibiltity that the Nuget team will decide to either change the schema or even migrate to a new file format. Hopefully we can just update the `Nuget` nuggets if that happens.

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[Xamarin.Android.Build.Tasks] Better support for netstandard libraries. #1356

[Xamarin.Android.Build.Tasks] Better support for netstandard libraries. #1356

Commits on Mar 21, 2018