refactor: move testfixtures to internal package and update imports

- Relocated testfixtures from the root to the internal package for better encapsulation.
- Updated import paths in codec test files across env, ini, json, yaml directories to reflect the new location.
- Removed the now obsolete testfixtures.go file.

env/codec_test.go

@@ -4,7 +4,7 @@ import (
 	"context"
 	"testing"
 
-	"github.com/eslider/go-config/testfixtures"
+	"github.com/eslider/go-config/internal/testfixtures"
 )
 
 type identityRoot struct {

ini/codec_test.go

@@ -4,7 +4,7 @@ import (
 	"context"
 	"testing"
 
-	"github.com/eslider/go-config/testfixtures"
+	"github.com/eslider/go-config/internal/testfixtures"
 )
 
 func TestCodec_IdentityINIFixture(t *testing.T) {

json/codec_test.go

@@ -4,7 +4,7 @@ import (
 	"context"
 	"testing"
 
-	"github.com/eslider/go-config/testfixtures"
+	"github.com/eslider/go-config/internal/testfixtures"
 )
 
 func TestCodec_IdentityJSONFixture(t *testing.T) {

yaml/codec_test.go

@@ -7,7 +7,7 @@ import (
 
 	"github.com/eslider/go-config/internal/keymap"
 	"github.com/eslider/go-config/internal/merge"
-	"github.com/eslider/go-config/testfixtures"
+	"github.com/eslider/go-config/internal/testfixtures"
 	yaml3 "gopkg.in/yaml.v3"
 )
 

yaml/error_test.go

@@ -4,7 +4,7 @@ import (
 	"context"
 	"testing"
 
-	"github.com/eslider/go-config/testfixtures"
+	"github.com/eslider/go-config/internal/testfixtures"
 )
 
 func TestCodec_InvalidYAML(t *testing.T) {