This commit makes the current Terraform state version 3 (previously 2),
and a migration process as part of reading v2 state. For the most part
this is unnecessary: helper/schema will deal with upgrading state for
providers written with that framework. However, for providers which
implemented the resource model directly, this gives a best-efforts
attempt at lossless upgrade.
The heuristics used to change the count of a map from the .# key to the
.% key are as follows:
- if the flat map contains any non-numeric keys, we treat it as a
map
- if the map is empty it must be computed or optional, so we remove
it from state
There is a known edge condition: maps with all-numeric keys are
indistinguishable from sets without access to the schema. They will need
manual conversion or may result in spurious diffs.
