* addrs: detect builtin provider when parsing legacy provider string
The ParseLegacyAbsProviderConfig was not detecting builtin providers
("terraform"), which caused issues for all users with 0.12 state and
the "terraform_remote_state" data source. Since "terraform" is the only
built-in provider this adds a very simple check to the parser so it
properly returns the builtin FQN.
* add tests to the addrs package
a large refactor to addrs.AbsProviderConfig, embedding the addrs.Provider instead of a Type string. I've added and updated tests, added some Legacy functions to support older state formats and shims, and added a normalization step when reading v4 (current) state files (not the added tests under states/statefile/roundtrip which work with both current and legacy-style AbsProviderConfig strings).
The remaining 'fixme' and 'todo' comments are mostly going to be addressed in a subsequent PR and involve looking up a given local provider config's FQN. This is fine for now as we are only working with default assumption.
Kristin Laemmert