* 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.
These new types are adaptations of terraform.ProviderResolver and
terraform.ResourceProviderFactory respectively. In a later commit we will
update users of the old types to use these new types, possibly in a
"gradual repair" sort of fashion by initially using type aliases, though
that won't 100% solve the problem because the types themselves have
changed in this move to return providers.Interface instead of
terraform.ResourceProvider.
Kristin Laemmert