[Directory.Build.props] Set a dummy PackageVersion to work around VS/NuGet issue

[jpobst]

Context: NuGet/Home#6461

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

Error NETSDK1005 Assets file 'C:\code\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, that causes a problem because the package can't have multiple versions in it.

We can set a dummy $(PackageVersion) because we don't actually need it.

Also updates the generator.slnf for additional needed projects to build generator.

[jonathanpeppers]

I guess we'd change this if we ever ship a NuGet package from this repo? Maybe wouldn't happen.

[jpobst]

I guess we could drop this hack when we no longer target classic.

[jonpryor]

If (when?) we ever ship a NuGet package from this repo, it should presumably match the version of Java.Interop.dll, which will match the .NET version it's shipping with (6.0.0.0, 7.0.0.0, etc.).

Thus, perhaps we should set $(PackageVersion)=6.0.0.0?

[jpobst]

Given that we have no intentions of releasing a NuGet package from this repo, I suspect a 6.0.0.0 version number will be just as obsolete as 1.0.0.0 if we ever do ship a package. 😁