Provider default checks as parent-based

[guybedford]

Currently short of the providers configuration, providers always pick up from the defaultProvider. In more complex multi-provider scenarios, we should instead always pick up the provider context from the parent unless otherwise indicated through the providers config.

[Bubblyworld]

So in order of precedence, provider picking should follow:

1. there's a providers override for the parent context
2. the parent context belongs to a particular provider
3. the input map is mostly populated with a particular provider
4. the defaultProvider