internal/configs: Remove HCL-based functionality

The SDK doesn't work with HCL so the HCL-related parts of this package are
not necessary here, and removing them gives the freedom to switch away
from using cty as the value representation in a later commit.

The ImpliedType method was previously implemented in terms of the hcldec
package and the DecoderSpec method, so this includes a new version of
ImpliedType that just works directly with cty and doesn't go through the
hcldec ImpliedType function. The existing ImpliedType tests show no change
in behavior, and for good measure I added some type consistency checks
to the tests for two other methods that are unchanged by this commit that
show that the ImpliedType result remains compatible with them.

Author: apparentlymart

internal/configs/configschema/coerce_value_test.go

@@ -3,6 +3,7 @@ package configschema
 import (
 	"testing"
 
+	"github.com/davecgh/go-spew/spew"
 	"github.com/zclconf/go-cty/cty"
 
 	"github.com/hashicorp/terraform-plugin-sdk/v2/internal/tfdiags"
@@ -559,6 +560,14 @@ func TestCoerceValue(t *testing.T) {
 			if !gotValue.RawEquals(test.WantValue) {
 				t.Errorf("wrong result\ninput: %#v\ngot:   %#v\nwant:  %#v", test.Input, gotValue, test.WantValue)
 			}
+
+			// The coerced value must always conform to the implied type of
+			// the schema.
+			wantTy := test.Schema.ImpliedType()
+			gotTy := gotValue.Type()
+			if errs := gotTy.TestConformance(wantTy); len(errs) > 0 {
+				t.Errorf("empty value has incorrect type\ngot: %#v\nwant: %#v\nerrors: %s", gotTy, wantTy, spew.Sdump(errs))
+			}
 		})
 	}
 }