Support references to external types

[lukehoban]

Pulumi schema can contain references to externally defined types from other packages. This does not yet appear to be supported by this library. In principal it seems like it could be supported either automatically (if package name and version can be identified?) or via some manual mapping information users provide?

This is to support something like this:

"properties": {
                "functions": {
                    "type": "array",
                    "items": {
                        "$ref": "/aws/v4.0.0/schema.json#/resources/aws:lambda%2Ffunction:Function"
                    },
                    "description": "The functions managed by this service."
                },
                "url": {
                    "type": "string",
                    "description": "The API Gateway REST API managed by this service, if any of the functions expose HTTP handlers."
                }
            },

Via something like this:

type Service struct {
	pulumi.ResourceState

	// Outputs
	Url     pulumi.StringPtrOutput `pulumi:"url" provider:"output"`
	Lambdas []*lambda.Function     `pulumi:"lambdas" provider:"output"`

	// Inputs
	Functions FunctionMapInput     `pulumi:"functions"`
	Provider  ServiceProviderInput `pulumi:"provider"`
}

[iwahbe]

We can already do this in theory for types like ServiceProviderOutput. We can implement support for this generically (input+output) by traversing the To*Output of FunctionMapInput and then calling ElementType on an empty instance of that. This should be a transparent experience to users.

[lukehoban]

#21 definitely addressed other issues (I made more progress on #17 👏 ) - but didn't address the issue described in this issue.

For two reasons I believe:

1. The code noted above (and commented out in Attempt to port a serverless component to this library #17) triggers panic: Detected pointer type during serialization - did you forget to dereference?.
2. I'm fairly sure even if that was fixed or worked around, this would run into what's captured in Ensure we generate valid tokens for external types with non-standard tokens #20?

If (2) is the only issue, this could be closed as a dupe of #20, but probably worth reopening this for now? (I can't as I'm not a contributor on this repo 😄).

[hliuson]

I believe this issue is that []*lambda.Function is an array of pointers, not that it's an external type. I've opened a pull request (#23 ) which should address this.

[lukehoban]

Fixed with #23.