[Directory.Build.props] Set dummy $(PackageVersion) to appease NuGet (#971)

Context: NuGet/Home#6461

Trying to build `Java.Interop.csproj` in Visual Studio currently
fails with several errors such as:

	Error NETSDK1005:
	Assets file 'C:\…\xamarin-android\external\Java.Interop\tools\generator\obj\project.assets.json' 
	doesn't have a target for 'net6.0'.
	Ensure that restore has run and that you have included 'net6.0' in the TargetFrameworks for your project.

Even though we don't build NuGet packages, it still attempts to
calculate the NuGet package name, which includes the assembly
`$(Version)` if `$(PackageVersion)` isn't specified.

Because our assemblies have different versions per-TFM (cbd9666,
though note 3851b1a), that causes a problem because the package
can't have multiple versions in it.  

Instead, set a dummy `$(PackageVersion)` value because we don't
actually need it.

Additionally, update `generator.slnf` for additional needed projects
to build `generator`.

Author: jpobst

Directory.Build.props

@@ -8,6 +8,15 @@
     <_OutputPath>$(MSBuildThisFileDirectory)bin\Build$(Configuration)\</_OutputPath>
     <GenerateAssemblyInfo>true</GenerateAssemblyInfo>
     <ProduceReferenceAssemblyInOutDir>true</ProduceReferenceAssemblyInOutDir>
+    
+    <!-- 
+      Workaround for https://github.com/NuGet/Home/issues/6461 (VSWin Only)
+      Even though we don't build NuGet packages, it still attempts to build the NuGet package name, which includes 
+      the assembly version if PackageVersion isn't specified. Because our assemblies have different versions
+      per-TFM, that causes a problem because the package can't have multiple versions in it.  
+      Set a dummy PackageVersion because we don't actually need it.
+    -->
+    <PackageVersion>1.0.0.0</PackageVersion>
   </PropertyGroup>
 
   <PropertyGroup>

tools/generator/generator.slnf

@@ -2,11 +2,13 @@
   "solution": {
     "path": "..\\..\\Java.Interop.sln",
     "projects": [
+      "external\\xamarin-android-tools\\src\\Xamarin.Android.Tools.AndroidSdk\\Xamarin.Android.Tools.AndroidSdk.csproj",
       "src\\Java.Interop.Localization\\Java.Interop.Localization.csproj",
       "src\\Java.Interop.Tools.Cecil\\Java.Interop.Tools.Cecil.csproj",
       "src\\Java.Interop.Tools.Diagnostics\\Java.Interop.Tools.Diagnostics.csproj",
       "src\\Java.Interop.Tools.Generator\\Java.Interop.Tools.Generator.csproj",
       "src\\Java.Interop.Tools.JavaCallableWrappers\\Java.Interop.Tools.JavaCallableWrappers.csproj",
+      "src\\Java.Interop.Tools.JavaSource\\Java.Interop.Tools.JavaSource.csproj",
       "src\\Java.Interop.Tools.JavaTypeSystem\\Java.Interop.Tools.JavaTypeSystem.csproj",
       "src\\Xamarin.Android.Tools.AnnotationSupport\\Xamarin.Android.Tools.AnnotationSupport.csproj",
       "src\\Xamarin.Android.Tools.ApiXmlAdjuster\\Xamarin.Android.Tools.ApiXmlAdjuster.csproj",