When global.json specifies an SDK that can't be found, don't block project load in Visual Studio

[dsplaisted]

As described in the "Global.json failures" section of this spec: dotnet/designs#104

We would update the SDK resolver so that when running in Visual Studio, if the requested SDK from global.json is not found, it would fall back to using the SDK that would have been resolved if there was no global.json. It will also set properties or include items indicating that the specified SDK couldn't be found. The SDK will then add a target that will fail the build if it detects these properties or items, giving information about why the resolution failed.

We may need to add the ability for resolvers to read global properties in order for the resolver to know that it's running in VS.

[Nirmal4G]

Why not allow something like...

"msbuild-sdks":
{
    "search-paths": [ "src/", "build/sdks/", "env:tmp/sdks", "//server/share/sdks" ]
}

The above pattern will be useful in testing the SDKs locally. Order does matter, obviously! 😁