Add a single global schema cache for providers. This allows multiple
provider instances to share a single copy of the schema, and prevents
loading the schema multiple times for a given provider type during a
single command.
This does not currently work with some provider releases, which are
using GetProviderSchema to trigger certain initializations. A new server
capability will be introduced to trigger reloading their schemas, but
not store duplicate results.
This is most easily handled in the plugin code, without involving
Terraform core.
The biggest change here other than checking the PlanDestroy capability,
is the removal of the schema helper methods in the plugins. With the
addition of the capabilities field, combined with the necessity of
checking diagnostics from the schema, the helpers have outlived their
usefulness. Perhaps there's a better pattern for these repetitive calls,
but for now there isn't too extra verbosity involved.
We can no longer be assured that the particular instance of a provider
we are using has had GetProviderSchema called. Always check the
diagnostics even if we're fetching a cached response.
This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.
If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.
