If a configuration had multiple blocks in the versions.tf file, it would
be added to the `rewritePaths` list multiple times. We would then remove
it from this slice, but only once, and so the output file would later be
rewritten to remove the required providers block.
This commit uses a set instead of a list to prevent this case, and adds
a regression test.
Instead of using providers.tf as the default output file for the
upgrader, we now default to versions.tf. This means that if the
configuration has no `required_providers` blocks at all, or has
multiple, the provider version requirements will be stored in the
versions.tf file.
We now also update the versions.tf file to set a `required_version`
attribute in the first `terraform` block, with value ">= 0.13". This
is similar to the behaviour of the 0.12upgrade command, and signals that
the configuration should not be used with older versions of Terraform.
Alisdair McDiarmid