Allow runtime-specific assets in the project

[bricelam]

Ideally, I would like to add the following directly to my *.csproj and have the correct native library used.

<ItemGroup>
  <None Update="runtimes\linux-x64\native\libsqlite3.so">
    <Pack>true</Pack>
    <PackagePath>\</PackagePath>
  </None>
  <None Update="runtimes\osx-x64\native\libsqlite3.dylib">
    <Pack>true</Pack>
    <PackagePath>\</PackagePath>
  </None>
  <None Update="runtimes\win7-x86\native\sqlite3.dll">
    <Pack>true</Pack>
    <PackagePath>\</PackagePath>
  </None>
</ItemGroup>

Unfortunately, native assets only appear to work with NuGet packages and not projects (or project references).

cc @natemcmaster

[bricelam]

See also #765