Implement reference validation

Add the ability to use the collected origin and target references in early validation.

Validation funcs will be provided by terraform-ls for now.

internal/decoder/decoder.go

@@ -7,6 +7,7 @@ import (
 	"context"
 
 	"github.com/hashicorp/hcl-lang/decoder"
+	"github.com/hashicorp/hcl-lang/lang"
 	"github.com/hashicorp/hcl-lang/reference"
 	"github.com/hashicorp/hcl/v2"
 	"github.com/hashicorp/terraform-ls/internal/codelens"
@@ -91,5 +92,41 @@ func DecoderContext(ctx context.Context) decoder.DecoderContext {
 		}
 	}
 
+	// TODO: extract out of this file
+	dCtx.Validations = append(dCtx.Validations, func(ctx context.Context) lang.DiagnosticsMap {
+		// get files from pathContext
+		//iterate over files and validate
+		pathCtx, err := decoder.PathCtx(ctx)
+		if err != nil {
+			// TODO
+		}
+
+		diagsMap := make(lang.DiagnosticsMap)
+
+		for _, origin := range pathCtx.ReferenceOrigins {
+			matchableOrigin, ok := origin.(reference.MatchableOrigin)
+			if !ok {
+				// TODO: add a diag here
+				continue
+			}
+
+			_, ok = pathCtx.ReferenceTargets.Match(matchableOrigin)
+			if !ok {
+				// target not found
+				diagsMap[origin.OriginRange().Filename] = hcl.Diagnostics{
+					&hcl.Diagnostic{
+						Severity: hcl.DiagError,
+						Summary:  "No reference found", // TODO: Is there more we can state here?
+						Subject:  origin.OriginRange().Ptr(),
+					},
+				}
+				continue
+			}
+
+		}
+
+		return diagsMap
+	})
+
 	return dCtx
 }