Provider `function`s never automap during fallback stategy

[guineveresaenger]

During the signalfx upgrade I saw the following error:

error: failed to gather package metadata: problem gathering provider functions: 1 error occurred:
	* TF function "parse_time_range" not mapped to the Pulumi provider

This affects every unmapped function on any provider using automapping that triggers fallbackstrat.

For Signalfx, I added an explicit mapping to avoid hitting the bug.