Add limited net11/net30/net35/net403 support (mostly for NuGet packages)

[j3parker]

This an alternative to #66 (for #59 )

I think this is way less hacky.

[j3parker]

Without this, the error message is:

The dependency @net//:StandardReferences is not compatible with net11!

I think this is a pretty good outcome!

[j3parker]

So the idea here is that we never "fill in" these frameworks with compatible ones (e.g. net20 -> net30) but these can be used to populate the other direction (net30 -> net40). The precedence is the natural order.

Maybe we don't need to do this. I wanted to prevent @net//:System.Linq from existing though, because that's funky. It does mean it'd be extra hard to build net30 things with csharp_library, but I don't know that we should really be worried about that.

[omsmith]

Oh yeah, true, having net20 provide the standard references for everything else wouldn't be amazing.

[omsmith]

Should we just fill out these edges? As you've shown, we're technically supporting building with them, just without the availability of the framework.

Does not having "net35": ["net30"] mean you can't build a net45 library, that depends on a net35 nuget, which itself depends on a net30 nuget?

[j3parker]

I was freaked out about accidentally creating @net35//:System.Linq. I don't feel strongly about it though.

Yeah it might. Ok I'll do the normal thing.

[j3parker]

You know, maybe import_multiframework_library could just signal what things it didn't want filled in or something. It seems like a minor issue though.

[omsmith]

Yeah, I was going to say, I think we just want something like a no_version_smearing or whatever flag on the rules, which we can use for the framework references.

[j3parker]

The only hits for net11 in the action graph, as desired:

2019-10-09T01:52:37.8286901Z action 'Compiling OldThing'
2019-10-09T01:52:37.8286996Z   Mnemonic: CSharpCompile
2019-10-09T01:52:37.8287088Z   Target: //import_nuget_package:OldThing
2019-10-09T01:52:37.8287328Z   Configuration: k8-fastbuild
2019-10-09T01:52:37.8287408Z   ActionKey: 49a152d61ad2aeb19e89ed71a6b79bdb
2019-10-09T01:52:37.8288306Z   Inputs: [external/ExamplePackageFolder/lib/net11/SomePackage.dll, external/csharp-build-tools/tasks/netcoreapp2.1/bincore/csc.dll, external/netcore-runtime-linux/dotnet, import_nuget_package/Class.cs]
2019-10-09T01:52:37.8289121Z   Outputs: [bazel-out/k8-fastbuild/bin/import_nuget_package/bazelout/net11/OldThing.dll, bazel-out/k8-fastbuild/bin/import_nuget_package/bazelout/net11/OldThing.pdb, bazel-out/k8-fastbuild/bin/import_nuget_package/bazelout/net11/OldThing.ref.dll]
2019-10-09T01:52:37.8289545Z   Command Line: (exec external/netcore-runtime-linux/dotnet \
2019-10-09T01:52:37.8289886Z     external/csharp-build-tools/tasks/netcoreapp2.1/bincore/csc.dll \
2019-10-09T01:52:37.8290010Z     /noconfig \
2019-10-09T01:52:37.8290286Z     /unsafe- \
2019-10-09T01:52:37.8290560Z     /checked- \
2019-10-09T01:52:37.8290667Z     /nostdlib+ \
2019-10-09T01:52:37.8290774Z     /utf8output \
2019-10-09T01:52:37.8290879Z     /deterministic+ \
2019-10-09T01:52:37.8290967Z     /filealign:512 \
2019-10-09T01:52:37.8291243Z     /nologo \
2019-10-09T01:52:37.8291331Z     /highentropyva+ \
2019-10-09T01:52:37.8291419Z     /warn:0 \
2019-10-09T01:52:37.8291514Z     /target:library \
2019-10-09T01:52:37.8291604Z     /langversion:7.3 \
2019-10-09T01:52:37.8291694Z     /debug+ \
2019-10-09T01:52:37.8291937Z     /optimize- \
2019-10-09T01:52:37.8292158Z     '/define:TRACE;DEBUG' \
2019-10-09T01:52:37.8292250Z     /debug:portable \
2019-10-09T01:52:37.8292541Z     /out:bazel-out/k8-fastbuild/bin/import_nuget_package/bazelout/net11/OldThing.dll \
2019-10-09T01:52:37.8292856Z     /refout:bazel-out/k8-fastbuild/bin/import_nuget_package/bazelout/net11/OldThing.ref.dll \
2019-10-09T01:52:37.8293167Z     /pdb:bazel-out/k8-fastbuild/bin/import_nuget_package/bazelout/net11/OldThing.pdb \
2019-10-09T01:52:37.8293284Z     /r:external/ExamplePackageFolder/lib/net11/SomePackage.dll \
2019-10-09T01:52:37.8293385Z     import_nuget_package/Class.cs \
2019-10-09T01:52:37.8293479Z     /d:NETFRAMEWORK \
2019-10-09T01:52:37.8293574Z     /d:NET11)
2019-10-09T01:52:37.8293615Z