* terraform/context: use new addrs.Provider as map key in provider factories
* added NewLegacyProviderType and LegacyString funcs to make it explicit that these are temporary placeholders
This PR introduces a new concept, provider fully-qualified name (FQN), encapsulated by the `addrs.Provider` struct.
Since there is little left that isn't core, remove the distinction for
now to reduce confusion, since a "core" binary will mostly work except
for provisioners.
Aaron Heesakkers