Built-in providers are special providers that are distributed as part of
Terraform CLI itself, rather than being installed separately. They always
live in the terraform.io/builtin/... namespace so it's easier to see that
they are special, and currently there is only one built-in provider named
"terraform".
Previous commits established the addressing scheme for built-in providers.
This commit makes the installer aware of them to the extent that it knows
not to try to install them the usual way and it's able to report an error
if the user requests a built-in provider that doesn't exist or tries to
impose a particular version constraint for a built-in provider.
For the moment the tests for this are the ones in the "command" package
because that's where the existing testing infrastructure for this
functionality lives. A later commit should add some more focused unit
tests here in the internal/providercache package, too.
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