Better error message when importing missing stack

Description of the change

The terraform import handling for stacks was reusing the resourceStackRead() function, which sets the ID to an empty string if no stack is found. To solve this I've pulled out query for a stack and updating the state based on a stack struct into separate functions, and I've updated the import handling to detect when the stack can't be found an output a decent error message.

I pulled the PopulateStack() function into the structs package to follow the pattern we've taken for the scheduled task structs.

Type of change

[x] Bug fix (non-breaking change that fixes an issue)
[ ] New feature (non-breaking change that adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
[ ] Documentation (non-breaking change that adds documentation)

Related issues

Fixes #405

Checklists

Development

[x] Lint rules pass locally
[x] The code changed/added as part of this pull request has been covered with tests
[x] All tests related to the changed code pass in development
[x] Examples for new resources and data sources have been added
[x] Default values have been documented in the description (e.g., "Dummy: (Boolean) Blah blah. Defaults to false.)
[ ] If the action fails that checks the documentation: Run go generate to make sure the docs are up to date

Code review

[x] This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached
[x] Pull Request is no longer marked as "draft"
[x] Reviewers have been assigned
[x] Changes have been reviewed by at least one other engineer
[x] The target branch is future unless the change is going directly into production

spacelift-io / terraform-provider-spacelift