core: Clarify the MissingProviderTransformer logic
core: Clarify the MissingProviderTransformer logic

The prior implementation of this function (before the rewrite to use
addrs.AbsProviderConfig values) was relying on some implicit behavior of
our provider address normalization to generate an address in the root
module. Since that wasn't explicit, I introduced a bug here when
introducing the new address type, where I generated an address in the
node's own module, rather than in the root as expected.

This new implementation is functionally equivalent to the prior, but is
written to make the intent more obvious: take _just_ the type from the
node's provider address and create an implicit configuration for it
_in the root module_.

Along with the change in approach, this new implementation also has an
updated documentation comment that better describes its current intent
(previously it was outdated) and hopefully clearer trace logging to
better communicate what it's doing.

VladRassokhin/terraform,pdecat/terraform,pradeepbhadani/terraform,tommynsong/terraform,mattray/terraform,luis-silva/terraform,hashicorp/terraform,nlamirault/terraform,luis-silva/terraform,iljaskevic/terraform,rnaveiras/terraform,VladRassokhin/terraform,mattray/terraform,drebes/terraform,tpounds/terraform,mattray/terraform,pdecat/terraform,apparentlymart/terraform,pdecat/terraform,apparentlymart/terraform,tommynsong/terraform,rnaveiras/terraform,gechr/terraform,iljaskevic/terraform,brandontosch/terraform,brandontosch/terraform,tpounds/terraform,gechr/terraform,tpounds/terraform,youhong316/terraform,apparentlymart/terraform,VladRassokhin/terraform,heimweh/terraform,VladRassokhin/terraform,heimweh/terraform,pradeepbhadani/terraform,brandontosch/terraform,youhong316/terraform,tommynsong/terraform,youhong316/terraform,youhong316/terraform,nlamirault/terraform,drebes/terraform,pryorda/terraform,pryorda/terraform,drebes/terraform,apparentlymart/terraform,tpounds/terraform,hashicorp/terraform,ewbankkit/terraform,brandontosch/terraform,drebes/terraform,pradeepbhadani/terraform,nlamirault/terraform,apparentlymart/terraform,ewbankkit/terraform,iljaskevic/terraform,mattray/terraform,gechr/terraform,ewbankkit/terraform,heimweh/terraform,luis-silva/terraform,pdecat/terraform,iljaskevic/terraform,hashicorp/terraform,pryorda/terraform,heimweh/terraform,ewbankkit/terraform,luis-silva/terraform,heimweh/terraform,brandontosch/terraform,rnaveiras/terraform