fix: Include backend in dependent bodies to avoid false negatives

[radeksimko]

Context

A bug was discovered as part of testing hashicorp/terraform-ls#1368 - see below.

---

The assumption I made in the beginning of investigating this bug - which is likely the same assumption made while the bug was introduced in the first place - that the backend attribute is part of a core schema which we'd end up keeping after merge, just like we keep depends_on and similar ones.

However, the terraform_remote_state data source is a bit different in that the "merge hierarchy" is much flatter, where the "non-backend" body is on the same level as the "backend-specific" body, so no merging can ever happen between the two:

// data block
DependentBody: {
     "{\"labels\":[{\"index\":0,\"value\":\"terraform_remote_state\"}]}": &schema.BodySchema{
                                Attributes: {
                                    "backend": &schema.AttributeSchema{
                                        IsRequired:   true,
                                        Constraint:   schema.OneOf{
                                            schema.LiteralValue{
                                                Value:        cty.StringVal("azurerm"),
                                                IsDeprecated: false,
                                                Description:  lang.MarkupContent{Value:"Azure Blob Storage", Kind:0x2},
                                            },
// ...
                                            schema.LiteralValue{
                                                Value:        cty.StringVal("s3"),
                                                IsDeprecated: false,
                                                Description:  lang.MarkupContent{Value:"Amazon S3 (with locking via DynamoDB)", Kind:0x2},
                                            },
                                        },
                                        IsDepKey:               true,
                                        SemanticTokenModifiers: {"hcl-dependent"},
                                    },
     "{\"labels\":[{\"index\":0,\"value\":\"terraform_remote_state\"}],\"attrs\":[{\"name\":\"backend\",\"expr\":{\"static\":\"s3\"}}]}": &schema.BodySchema{
                    Blocks:     {},
                    Attributes: {
// ... (no backend attribute here)

Implementation Notes

I initially thought we could just set the backend Constraint to the given schema.LiteralValue{} for each backend. While this would address the validation problem, it would also make it much more difficult to solve hashicorp/terraform-ls#1284 as we wouldn't have access to all the other backends and their details at completion time, when the merging has already taken place (because backend has matching value).

[dbanck]

👍 appreciate the detailed description