Do not convert provisioner diagnostics to errors so that users can get
context from provisioner failures.
Return diagnostics from the builtin provisioners that can be annotated
with configuration context and instance addresses.
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.
