* 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
58bcc2e9bb