Error on provider authors when they use pulumi's Asset type

[thomas11]

As discussed in #237, the core Asset doesn't roundtrip correctly, so we introduced types.AssetOrArchive in this provider.

Nothing prevents a provider author from still using Asset, though. We should at least print a warning at schemagen time when they do that.

I attempted this in a simple way in Warn about using the core Asset and Archive types but that yielded false positive warnings. The local.Command has Assets as an output property which is ok but triggered the warning.