The introduction of a heirarchical addressing scheme for providers gives
us an opportunity to make more explicit the special case of "built-in"
providers.
Thus far we've just had a special case in the "command" package that the
provider named "terraform" is handled differently than all others, though
there's nothing especially obvious about that in the UI.
Moving forward we'll put such "built-in" providers under the special
namespace terraform.io/builtin/terraform, which will be visible in the UI
as being different than the other providers and we can use the namespace
itself (rather than a particular name) as the trigger for our special-case
behaviors around built-in plugins.
We have no plans to introduce any built-in providers other than
"terraform" in the foreseeable future, so any others will produce an
error.
This commit just establishes the addressing convention, without making use
of it anywhere yet. Subsequent commits will make the provider installer
and resolver codepaths aware of it, replacing existing checks for the
provider just being called "terraform".
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