Flow transitive build targets for project references

[JunTaoLuo]

Right now the build targets that override LiteClientBase is only included in Grpc.Net.Client. The packages that transitively depend on this package (such as Grpc.Net.ClientFactory) do not flow those targets so referencing them would lead to type missing errors at build time (since the override did not occur and the base class resolves to ClientBase instead).

Though it's documented at https://docs.microsoft.com/en-us/nuget/reference/msbuild-targets#project-to-project-references, PrivateAssets=All on a ProjectReference did not translate to a PrivateAssets=All in the nuspec during pack. As a fallback, I'm adding the override target to all packages that transitively reference Grpc.Net.Client. I've tested that this resolves the issue even when multiple of these packages are referenced together. Though it can be argued that users shouldn't do this, at least this doesn't lead to catastrophic failure.

I need to check with the nuget folks to see if there's a way to propagate the PrivateAssets settings on a project to project reference to the nuspec at pack time.

[jtattermusch]

LGTM, but I'd like to understand better why the PrivateAssets=None doesn't work.
Should we file an issue to investigate and remove the workaround once possible?

[JunTaoLuo]

Actually, it seems like the results of pack were being cached. PrivateAssets seems to work on a clean build. Will update PR.

I thought pack was also affected by dotnet/sdk#952 but upon closer inspection that's a totally unrelated issue.