If a user forgets to specify the source address for a provider, Terraform
will assume they meant a provider in the registry.terraform.io/hashicorp/
namespace. If that ultimately doesn't exist, we'll now try to see if
there's some other provider source address recorded in the registry's
legacy provider lookup table, and suggest it if so.
The error message here is a terse one addressed primarily to folks who are
already somewhat familiar with provider source addresses and how to
specify them. Terraform v0.13 had a more elaborate version of this error
message which directed the user to try the v0.13 automatic upgrade tool,
but we no longer have that available in v0.14 and later so the user must
make the fix themselves.
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