config: a nicer error message for invalid provider constraints

Previously our error message here was confusing and redundant:

    Error starting operation: provider.null: invalid version constraint "not valid": Malformed constraint: not valid

Instead, we'll generate a full HCL2 diagnostic here, which results in
something (subjectively) nicer:

    Error: Invalid provider version constraint

    The value "@ 1.0.0" given for provider.null is not a valid version
    constraint.

At the moment this message is an outlier in that the other validation
errors are all still just plain Go errors, but over time we'll want to
adjust all of these to be full diagnostics so that we can embed source
range information in them to help the user find the offending
configuration.

config/config.go

@@ -8,6 +8,7 @@ import (
 	"strconv"
 	"strings"
 
+	hcl2 "github.com/hashicorp/hcl2/hcl"
 	"github.com/hashicorp/hil/ast"
 	"github.com/hashicorp/terraform/helper/hilmapstructure"
 	"github.com/hashicorp/terraform/plugin/discovery"
@@ -415,10 +416,17 @@ func (c *Config) Validate() tfdiags.Diagnostics {
 		if p.Version != "" {
 			_, err := discovery.ConstraintStr(p.Version).Parse()
 			if err != nil {
-				diags = diags.Append(fmt.Errorf(
-					"provider.%s: invalid version constraint %q: %s",
-					name, p.Version, err,
-				))
+				diags = diags.Append(&hcl2.Diagnostic{
+					Severity: hcl2.DiagError,
+					Summary:  "Invalid provider version constraint",
+					Detail: fmt.Sprintf(
+						"The value %q given for provider.%s is not a valid version constraint.",
+						p.Version, name,
+					),
+					// TODO: include a "Subject" source reference in here,
+					// once the config loader is able to retain source
+					// location information.
+				})
 			}
 		}
 

config/config_test.go

@@ -217,7 +217,7 @@ func TestConfigValidate_table(t *testing.T) {
 			"provider with invalid version constraint",
 			"provider-version-invalid",
 			true,
-			"invalid version constraint",
+			"not a valid version constraint",
 		},
 		{
 			"invalid provider name in module block",