Integrate tflint with focus on a generalized linter/validator interface

[radeksimko]

This is part of hashicorp/vscode-terraform#635

Use-cases

(Follow up on #27)
The user can run terraform validate since 0.11 as a command (if dedicated clients such as VS Code extension choose to integrate) and can also run this on didSave automatically.

The integration of linters requires similar work and could be implemented as a command, which can be executed on didSave as well.

Proposal

1. Start with the integration of a single linter tflint
2. Make use of the settings introduced in Add linter configuration to initialization options #855
3. Implement tflint as an executable command, which calls the tflint binary, similar to validate
   

   terraform-ls/internal/langserver/handlers/command/validate.go

   Lines 57 to 65 in e7abb2f

   	progress.Begin(ctx, "Validating")
   	defer func() {
   	progress.End(ctx, "Finished")
   	}()
   	progress.Report(ctx, "Running terraform validate ...")
   	jsonDiags, err := tfExec.Validate(ctx)
   	if err != nil {
   	return nil, err
   	}

4. Inspect the output tflint --json and how to store it in the diagnostics memdb table introduced in Store diagnostics in memdb table #856
5. Add tflint to the didSave handler, similar to validate
   

   terraform-ls/internal/langserver/handlers/did_save.go

   Lines 18 to 29 in e7abb2f

   	if !expFeatures.ValidateOnSave {
   	return nil
   	}
   	dh := ilsp.HandleFromDocumentURI(params.TextDocument.URI)
   	cmdHandler := &command.CmdHandler{
   	StateStore: svc.stateStore,
   	}
   	_, err = cmdHandler.TerraformValidateHandler(ctx, cmd.CommandArgs{
   	"uri": dh.Dir.URI,
   	})

[wata727]

@dbanck I'm currently interested in working on this, here is the PoC:

• 5ed6f59
• hashicorp/vscode-terraform@950ba14

As I understand it, #856 has already been resolved, so there are no related fixes in the PoC.

If you are able to move this forward I would be happy to submit a PR, which can be split into separate PRs for each step outlined here, or combined. Please let me know if we need to discuss the overall design before we move forward.

Also, since the architecture will change significantly in #1667, it would be fine to implement it after that.