builder: add 'builtin' to builtin_module_parts instead of .clone()

vlib/v/builder/builder.v

@@ -18,13 +18,13 @@ pub:
 mut:
 	checker             checker.Checker
 	pref                &pref.Preferences
-	parsed_files        []ast.File
 	global_scope        &ast.Scope
 	out_name_c          string
 	out_name_js         string
 	max_nr_errors       int = 100
 pub mut:
 	module_search_paths []string
+	parsed_files        []ast.File
 }
 
 pub fn new_builder(pref &pref.Preferences) Builder {
@@ -142,8 +142,7 @@ pub fn (mut b Builder) resolve_deps() {
 
 // graph of all imported modules
 pub fn (b &Builder) import_graph() &depgraph.DepGraph {
-	mut builtins := util.builtin_module_parts.clone()
+	builtins := util.builtin_module_parts
-	builtins << 'builtin'
 	mut graph := depgraph.new_dep_graph()
 	for p in b.parsed_files {
 		mut deps := []string{}

vlib/v/builder/compile.v

@@ -156,6 +156,10 @@ Did you forget to add vlib to the path? (Use @vlib for default vlib)')
 }
 
 pub fn (v Builder) get_user_files() []string {
+	if v.pref.path == 'vlib/builtin' {
+		// get_builtin_files() has already added the builtin files:
+		return []
+	}        
 	mut dir := v.pref.path
 	v.log('get_v_files($dir)')
 	// Need to store user files separately, because they have to be added after

vlib/v/util/util.v

@@ -12,7 +12,7 @@ pub const (
 
 // math.bits is needed by strconv.ftoa
 pub const (
-	builtin_module_parts = ['math.bits', 'strconv', 'strconv.ftoa', 'hash.wyhash', 'strings']
+	builtin_module_parts = ['math.bits', 'strconv', 'strconv.ftoa', 'hash.wyhash', 'strings', 'builtin']
 )
 
 pub const (