Require scl files in ExecutionModeScl

Also remove json from scl file builtins, as it's not in bazel.

Bug: 309686282
Test: Presubmits
Change-Id: I6227c5dc14922b4682380254b4716b4ae38c47b9

tools/rbcrun/host.go

@@ -24,7 +24,6 @@ import (
 	"strings"
 
 	"go.starlark.net/starlark"
-	"go.starlark.net/starlarkjson"
 	"go.starlark.net/starlarkstruct"
 )
 
@@ -60,7 +59,6 @@ var rbcBuiltins starlark.StringDict = starlark.StringDict{
 
 var sclBuiltins starlark.StringDict = starlark.StringDict{
 	"struct":   starlark.NewBuiltin("struct", starlarkstruct.Make),
-	"json": starlarkjson.Module,
 }
 
 func isSymlink(filepath string) (bool, error) {
@@ -389,6 +387,10 @@ func Run(filename string, src interface{}, mode ExecutionMode, allowExternalEntr
 		return nil, nil, err
 	}
 
+	if mode == ExecutionModeScl && !strings.HasSuffix(filename, ".scl") {
+		return nil, nil, fmt.Errorf("filename must end in .scl: %s", filename)
+	}
+
 	// Add top-level file to cache for cycle detection purposes
 	moduleCache[filename] = nil